The Synod of Constantinople 879–880 occupies an ambiguous and theologically charged place in the history of Christian conciliarity. For some traditions of reception, it represents the restoration of ecclesial peace after the conflicts surrounding the deposition and rehabilitation of Photius, patriarch of Constantinople; for others, it remains overshadowed by the council of 869–870, later received in the Latin West as the Eighth Ecumenical Council (Dvornik 1948:196–236; Perrone 1993:169–196; Stiernon 1975:221–233). Yet this very ambiguity makes the synod an unusually fruitful object of theological analysis. It stands at the intersection of several questions that remain decisive for ecclesiology: what constitutes ecclesial communion, who has authority to restore or rupture it, how the inherited Creed is protected, how the boundaries of orthodoxy are recognised and how primacy and synodality are to be related within the visible structure of the Church (Chrysos 2023: 155–170; Meijer 1975:123–141; Stephanou 1973:363–407).

The present article argues that the Synod of Constantinople 879–880 should be read not merely as an episode in the Photian controversy, but as a witness to an ecclesiology of communion. In this context, ‘communion’ does not mean vague spiritual harmony or a diplomatic settlement between ecclesiastical centres. It denotes a doctrinally, canonically and sacramentally structured reality in which bishops, local churches and patriarchal sees are bound together by the confession of the same faith, the recognition of legitimate ecclesial order and the restoration of peace where communion has been wounded (Chadwick 2003: 95–130; Dvornik 1966:100–118; Meijer 1975:74–122). The horos and canons of 879–880, therefore, deserve theological attention as more than documentary remains of a political reconciliation. They articulate a grammar of ecclesial belonging.

The source basis for this study is the conciliar material preserved in Conciliorum Oecumenicorum Generaliumque Decreta, where the Synod of Constantinople 879–880 is presented with an introduction, bibliography, conspectus of sigla, the Greek horos and three canons (eds. Alberigo & Melloni 2016; Gemeinhardt 2016:25–39). The dossier explicitly frames the council as an event that brought to an end a prolonged dispute between Constantinople and Rome, involving the status of Photius, the relation between the patriarchal sees, the Bulgarian question and the legitimacy of preserving the Nicene-Constantinopolitan Creed without unauthorised additions (Gemeinhardt 2016:25–32). This source base is especially important because the Greek horos and canons allow the synod to be examined not only through later polemical receptions, but through its own conciliar language (Gemeinhardt 2016:37–39).

The problem is not only historical. The Synod of Constantinople 879–880 raises a fundamental theological question: how does the Church define the limits of communion without reducing orthodoxy either to institutional obedience or to abstract doctrinal correctness? The council’s answer, as will be argued, is neither purely juridical nor purely dogmatic. It is ecclesiological. Communion is restored through recognition, but recognition itself is normed by the inherited faith. The Creed is defended not as a textual relic, but as the common rule of ecclesial identity. Canonical order is affirmed not as administrative control, but as the visible form of communion (Gemeinhardt 2002:41–68; Grumel 1938:357–372, 1947:218–234). The reception of Photius is therefore inseparable from the larger question of how the Church discerns legitimate belonging.

The article follows the methodological pattern of a historically grounded theological reading. It does not attempt to provide a full narrative history of the Photian conflict, nor does it treat the synod simply as a source for East–West polemics. Rather, it asks what theological vision of the Church is implied by the horos and canons. In this respect, the study adopts a method analogous to the theological reading of a historical phenomenon as a locus theologicus [theological locus]: a structured source through which theological claims may be responsibly articulated (Słomski 2026:1–3). In the HTS Teologiese Studies/Theological Studies model article, hesychasm is interpreted as a locus theologicus when experience is placed within a disciplined ecclesial and epistemological framework (Słomski 2026:1–3). Here, the Synod of Constantinople 879–880 is approached analogously: not as a mere event, but as a theological locus in which communion, orthodoxy and conciliar authority become visible.

The central thesis is that Constantinople 879–880 presents the Church as a communion constituted by three inseparable elements: fidelity to the inherited confession of faith, recognition of canonical order and conciliar restoration of ecclesial peace. The horos safeguards the Creed as the doctrinal boundary of communion; the canons regulate ecclesiastical discipline as the canonical form of communion; and the rehabilitation of Photius manifests the possibility of restored communion through conciliar recognition (Dragas 1999:357–369; Meijer 1975:123–141; Stephanou 1973:363–407). These three dimensions together disclose an ecclesiology in which orthodoxy is neither a private theological opinion nor merely the decision of one ecclesiastical centre, but the shared confession of the Church received, guarded and enacted through synodal discernment.

The present study adopts a historical-theological and hermeneutical method. Its primary object is not the entire Photian controversy, but the theological logic of the Synod of Constantinople 879–880 as expressed in its horos, canons and early reception. The article, therefore, proceeds by reading the conciliar texts as normative theological discourse rather than as neutral administrative records. This does not mean detaching them from their political and ecclesiastical setting; rather, it means asking how that setting is transformed into a theological grammar of communion, recognition and doctrinal boundary-making (Dvornik 1948:216–236; Gemeinhardt 2016:25–39; Meijer 1975:74–122).

The first methodological layer is source-critical. The study relies on the conciliar dossier transmitted in Conciliorum Oecumenicorum Generaliumque Decreta, where the Synod of Constantinople 879–880 is presented with an introduction, bibliography, conspectus of sigla, the Greek horos and three canons (eds. Alberigo & Melloni 2016; Gemeinhardt 2016:25–39). The scanned source confirms that the section contains the Greek horos and the canons under the title Concilium Constantinopolitanum 879–880, with reference to earlier editions, including Joannou, Hardouin and Mansi. This textual basis is crucial because it allows the synod to be approached through its own conciliar language rather than only through later Latin or Byzantine polemical memories.

The second methodological layer is historical contextualisation. The Synod of 879–880 cannot be interpreted apart from the sequence of ecclesiastical conflicts that began with the deposition of Ignatius, the elevation of Photius, the competing Roman and Constantinopolitan synods, the council of 869–870 and the eventual restoration of Photius under John VIII (Dvornik 1948:101–236; Polidori 2024:35–42; Stiernon 1975:221–233). Polidori’s recent historiographical synthesis is particularly useful because it stresses the multiplicity of synods between 859 and 880 and warns against reducing the matter to a simplified opposition between Rome and Constantinople (Polidori 2024:35–37).

The third methodological layer is ecclesiological interpretation. The article asks what concept of the Church is presupposed when communion is broken, restored, recognised or refused. This requires attention to the relationship between confession of faith, episcopal order, patriarchal recognition and canonical discipline. In this respect, the article follows the line of scholarship that has treated the Photian councils not only as political events but as expressions of competing ecclesiological models (Chrysos 2023:155–170; Meijer 1975:123–141; Stephanou 1973:363–407). Chrysos’ analysis of pentarchy is especially relevant, because it locates the councils of 869–870 and 879–880 within a broader debate about the structure of ecclesial authority and the symbolic weight of the five patriarchal sees (Chrysos 2023:155–170).

Several delimitations must be stated. Firstly, the article does not attempt to resolve the confessional dispute over whether Constantinople 879–880 should be recognised as ecumenical. That question belongs to the history of reception and remains differently answered in Orthodox and Roman Catholic traditions (Dragas 1999: 357–369; Dvornik 1966:314–328; Stiernon 1975:221–233). Secondly, the study does not provide a complete political history of ninth-century Byzantium or the Bulgarian mission. These matters are considered only insofar as they illuminate the synod’s ecclesiological logic (Betti 2014:102–148; Dvornik 1948:216–236). Thirdly, the article does not treat the Filioque [and from the Son] controversy in its full doctrinal development, but only insofar as the question of addition to the Creed becomes a criterion for the boundaries of communion (Gemeinhardt 2002:41–68; Grumel 1938:357–372; Siecienski 2010:75–108).

The working assumption is that conciliar texts are not merely descriptive sources but theological acts. A horos does not simply report a consensus; it performs the act of defining, guarding and transmitting ecclesial identity. Likewise, canons do not merely regulate external order; they disclose the Church’s understanding of communion as embodied in discipline, jurisdiction and recognition. This assumption allows the Synod of Constantinople 879–880 to be interpreted as a locus theologicus: a historically situated but theologically meaningful place in which the Church’s self-understanding becomes articulate (Meijer 1975:123–141; Słomski 2026:1–3).

The Synod of Constantinople 879–880 emerged from one of the most complex ecclesiastical crises of the ninth century. The conflict was not reducible to a personal rivalry between Ignatius and Photius, although that rivalry provided its immediate institutional form. It involved questions of episcopal deposition, patriarchal succession, papal intervention, imperial authority, missionary jurisdiction in Bulgaria and the theological status of the Nicene-Constantinopolitan Creed (Chadwick 2003:95–130; Dvornik 1948:101–236; Polidori 2024:35–42).

The standard polemical narrative long interpreted Photius as the principal architect of the so-called Eastern schism. Dvornik’s reconstruction radically challenged this view by showing that much of the anti-Photian tradition was shaped by later polemics and by the reception of sources hostile to Photius (Dvornik 1948:1–38, 216–236). Subsequent scholarship has not removed all controversy, but it has made clear that the councils of 869–870 and 879–880 must be read within a shifting network of ecclesiastical recognitions rather than through a simple opposition between papal legality and Byzantine resistance (Meijer 1975:74–122; Polidori 2024:35–39; Thümmel 2005:437–458).

The council of 869–870 condemned Photius and restored Ignatius. In later Latin reception, it came to be counted as the Eighth Ecumenical Council, but its historical status and representativeness have been repeatedly debated (Chrysos 2020: 139–160; Perrone 1993:169–196; Stiernon 1975:221–233). The later synod of 879–880, by contrast, rehabilitated Photius and restored communion between Rome and Constantinople under conditions accepted by the papal legates of John VIII (Dvornik 1948:216–236; Gemeinhardt 2016:25–32; Meijer 1975:74–122). The introduction in the conciliar dossier explicitly presents the 879–880 council as ending a dispute between Rome and Constantinople that had lasted for more than 20 years (Gemeinhardt 2016:25).

This restoration is theologically important because communion is not presented as a mere return to institutional normality. The issue is not only whether Photius personally may be recognised, but how ecclesial recognition itself operates. If communion can be broken by deposition, condemnation and refusal of recognition, it can also be restored through synodal judgement, mutual recognition and reaffirmation of the common faith. Thus, the synod treats communion as a visible and ecclesially mediated reality. It is neither purely invisible nor reducible to juridical submission (Meijer 1975:123–141; Stephanou 1973:363–407).

The Bulgarian question intensified this ecclesiological problem. The dispute over Bulgaria concerned more than territorial administration; it raised the issue of whether missionary jurisdiction and ecclesial dependency could be determined by Rome, Constantinople or imperial-political circumstances (Betti 2014:102–148; Dvornik 1948:159–192; Polidori 2024:35–37). Within this context, the restoration of communion required a delicate balance: Rome’s role had to be acknowledged, Constantinople’s dignity had to be preserved, and the unity of the Church had to be reconstructed without allowing one local conflict to become a permanent rupture.

More specifically, the synod did not attempt to settle the Bulgarian question by means of a new doctrinal definition or by transforming a jurisdictional conflict into a dogmatic boundary. Pope John VIII’s instructions linked Photius’s recognition with the restoration of Roman claims in Bulgaria and with the cessation of Constantinopolitan ordinations there; this shows that Rome’s role was not ignored but was treated as a real element in the process of reconciliation (Dvornik 1948:209–211; Polidori 2024:56–60). At the same time, the council itself avoided making Bulgaria the criterion of ecclesial communion. The papal legates accepted the conciliar restoration of Photius, while the practical question of Bulgarian jurisdiction remained bound to diplomatic negotiation among Rome, Constantinople, the emperor and Boris-Michael. Constantinople’s dignity was therefore preserved in two ways: Photius was not required to confess guilt for a deposition he and his supporters regarded as unjust, and the council did not reduce the patriarchate of Constantinople to a passive object of Roman adjudication. Yet Rome’s dignity was likewise acknowledged, since papal legates participated in the conciliar settlement, the Roman demand concerning Bulgaria was formally present in the negotiations and John VIII’s subsequent letters presupposed that the Roman see had obtained a meaningful concession, even if Bulgaria’s later development moved toward a more autonomous Slavic ecclesial identity (Dvornik 1948:210–214; Polidori 2024:60). The resolution was consequently not a simple victory of one see over another, but a fragile compromise: communion with Photius was restored synodally, Roman participation was publicly recognised, and the Bulgarian dispute was prevented from determining the whole ecclesiology of the council.

The Synod of 879–880 therefore reveals a key feature of early medieval ecclesiology: communion depends upon recognition, but recognition must itself be conciliar. The council does not imagine communion as a purely bilateral arrangement between Rome and Constantinople; it frames restoration within a gathered ecclesial body that includes representatives of patriarchal authority and articulates its judgement through a conciliar act (Chrysos 2023:155–170; Meijer 1975:74–122). This is why the synod is so significant for the theology of primacy and synodality. It neither abolishes primacy nor allows primacy to function apart from reception and conciliar discernment.

At this point, the ecclesiology of communion becomes visible. The Church is not simply a federation of local churches, nor a monarchy of one see over all others. It is a communion of churches whose unity is expressed through shared confession, mutual recognition and ordered conciliarity. The restoration of Photius is therefore not merely an administrative correction; it becomes the concrete case in which the Church negotiates how wounded communion may be healed without surrendering either doctrinal identity or canonical order (Chrysos 2023:155–170; Meijer 1975:123–141; Stephanou 1973:363–407).

The doctrinal centre of the Synod of Constantinople 879–880 is its horos. The text does not produce a new dogmatic formula in the manner of Nicaea or Chalcedon; rather, it protects the inherited confession of faith by excluding unauthorised alteration. This is precisely why the horos is ecclesiologically decisive. It shows that the boundary of orthodoxy is not created anew by each council, but received, guarded and transmitted within the continuity of the Church’s confession (Gemeinhardt 2016:29–32; Grumel 1938:357–372).

The conciliar dossier makes clear that the synod’s doctrinal concern is linked to the Nicene-Constantinopolitan Creed and to the controversy over additions to it. The introduction notes that previous research had debated whether the council’s dogmatic horos condemned any deviation from the text and teaching of the Creed, especially in relation to the Filioque (Gemeinhardt 2016:29–31; Grumel 1938:357–372; Jugie 1939:369–385). The Greek text of the horos, preserved in the scanned conciliar source, stands as the primary basis for this analysis.

The significance of the horos lies in the fact that the Creed is treated not merely as a doctrinal summary, but as a rule of communion. To alter the Creed is not simply to adjust a theological proposition; it is to disturb the common language through which the Church recognises itself as one body. In this sense, the prohibition of addition or subtraction has an ecclesiological force. The Creed functions as the boundary of orthodox communion because it is the shared confession by which churches, bishops and faithful are united in the same apostolic faith (Chadwick 2003:95–130; Gemeinhardt 2002:41–68; Grumel 1947:218–234).

This point is especially important for the Filioque controversy. The issue at stake in 879–880 is not yet identical with the later scholastic debate over the eternal procession of the Holy Spirit in its fully developed Latin and Greek forms. Rather, the immediate conciliar concern is the integrity of the Creed and the legitimacy of altering a text received as normative by the whole Church (Gemeinhardt 2002:41–68; Grumel 1930:257–264, 1938:357–372). The later doctrinal controversy cannot be ignored, but the synod’s own grammar is first of all conciliar and ecclesiological: no local or regional authority may unilaterally transform the common symbol of faith.

The horos, therefore, discloses a theology of reception. The Creed is authoritative not because it is a private text possessed by one see, nor because it is merely ancient, but because it has been received as the common confession of the Church. Its authority is conciliar, liturgical and ecclesial. It is recited, guarded and transmitted as a form of shared faith. The synod’s insistence on preserving the Creed thus expresses a principle that may be called doctrinal communion: the Church remains one by confessing one faith in a form that is not subject to unilateral alteration (Gemeinhardt 2016:29–32; Meijer 1975:123–141).

This does not mean that doctrinal development is impossible. The history of the ecumenical councils shows that the Church has often clarified, expanded and defended its confession in response to heresy and controversy. Yet, Constantinople 879–880 distinguishes between conciliar clarification and unauthorised interpolation. The council’s concern is not with theological reflection as such, but with the transformation of the Creed’s received form without the consent of the whole Church (Gemeinhardt 2002:41–68; Grumel 1938:357–372). This distinction is essential for understanding the synod’s ecclesiology. Orthodoxy is living and interpretive, but it is not arbitrary.

The horos also reveals the relation between doctrine and peace. The synod seeks reconciliation between Rome and Constantinople, but not by suspending doctrinal questions. Peace is not achieved by relativising the Creed; rather, communion is restored by returning to the shared confession. This is a crucial point. The council does not oppose unity and truth. It assumes that ecclesial peace must be grounded in doctrinal fidelity (Meijer 1975:74–122; Stephanou 1973:363–407). In this respect, the horos offers a model of reconciliation that is neither diplomatic compromise nor confessional rigidity, but conciliar return to the common rule of faith.

Theologically, the horos may therefore be read as an act of boundary-making. It marks the limits within which ecclesial communion remains recognisable. A boundary, however, need not be understood primarily as an exclusionary wall. In the conciliar grammar of 879–880, boundary means form: the form that allows the Church to remain itself across local, linguistic and political differences. The Creed binds Rome, Constantinople, and the other patriarchal sees not because it abolishes their differences, but because it gives them a shared doctrinal identity (Chrysos 2023:155–170; Gemeinhardt 2016:29–32).

This interpretation also guards against a reduction of the synod to anti-Latin polemic. Later Orthodox reception often emphasised Constantinople 879–880 as a condemnation of the Filioque addition, while Latin reception frequently diminished or relativised the council because of its Photian context (Dragas 1999:357–369; Dvornik 1948:216–236; Gemeinhardt 2002:41–68). Yet the synod itself can be read more fundamentally as an attempt to preserve the conciliar character of doctrine. Its ecclesiological claim is that the faith of the Church cannot be altered by unilateral action, even when such action is defended by local theological or liturgical custom.

In this way, the horos becomes a witness to the ecclesiology of communion. Communion requires a common faith; common faith requires a recognised confession; and the recognised confession must be guarded by the whole Church through conciliar discernment. The Creed is therefore not an isolated doctrinal artefact, but the living boundary of ecclesial identity (Gemeinhardt 2016:29–32; Meijer 1975:123–141). This is why the Synod of Constantinople 879–880 remains theologically significant beyond the immediate Photian conflict.

If the horos articulates the doctrinal boundary of communion, the canons express its canonical form. The Synod of Constantinople 879–880 did not confine itself to the question of the Creed or to the rehabilitation of Photius. It also issued disciplinary regulations concerning ecclesial order, episcopal conduct and the limits of ecclesiastical authority. These canons show that communion is not merely a doctrinal agreement, but an ordered life of the Church embodied in recognisable structures of discipline and responsibility (Gemeinhardt 2016:32–39; Meijer 1975:123–141).

The conciliar dossier preserves three canons following the horos. The scanned source shows the Greek text under the heading Kanones [Canons], immediately after the definition. This placement is theologically meaningful. Doctrine and discipline are not presented as unrelated domains. The horos defines the doctrinal form of communion; the canons regulate the ecclesial practices through which that communion is maintained. In the logic of the synod, orthodoxy and order belong together.

The first canon concerns those who had been condemned by Rome and Constantinople and regulates the ecclesiastical consequences of such condemnations. The issue is not merely punitive. The canon addresses the problem of how ecclesial judgements are recognised across different ecclesiastical centres and how communion is preserved when disciplinary decisions affect the wider Church (Gemeinhardt 2016:37–38). In this respect, the canon presupposes that local disciplinary action can have interecclesial consequences. A bishop or cleric is not simply a private member of one local structure; his status affects communion more broadly.

The second canon concerns monks who seek episcopal office or who abandon monastic discipline for ecclesiastical advancement. Its theological importance lies in its defence of vocation, stability and ecclesial integrity. The canon resists the instrumentalisation of monastic life as a path to power and thereby protects the Church from a confusion of spiritual discipline and hierarchical ambition (Gemeinhardt 2016:38–39). This is not a marginal matter. In Byzantine ecclesiology, monastic witness, episcopal authority and doctrinal fidelity were deeply interconnected. Disorder in one sphere could wound communion in another.

The third canon addresses violence or arbitrary action against bishops. It protects episcopal office from unjust deposition or coercion and therefore reflects one of the central wounds of the Photian conflict itself: the contested legitimacy of patriarchal deposition and restoration (Dvornik 1948:101–159; Gemeinhardt 2016:39; Meijer 1975:74–122). The canon is not merely defensive of episcopal privilege. It protects the stability of ecclesial order as a condition for communion. If bishops can be removed through factional violence or irregular pressure, the Church’s sacramental and canonical continuity is placed at risk.

Taken together, the canons display a coherent ecclesiological pattern. Communion requires recognised judgements, disciplined vocations and protection against arbitrary disruption of episcopal order. The Church is not simply held together by doctrine in abstraction; it is held together by practices of recognition, restraint and accountability. Canon law, in this framework, is not external to theology. It is theology in disciplinary form (Meijer 1975:123–141; Stephanou 1973:363–407).

This is particularly important for interpreting the relation between canon and communion. Modern readers often separate doctrinal theology from canon law, treating the former as concerned with truth and the latter as concerned with administration. The Synod of 879–880 resists such a separation. Its canons are not bureaucratic appendices to a doctrinal statement. They are expressions of the same ecclesial vision that animates the horos: the Church is a communion whose unity requires both shared confession and disciplined order (Gemeinhardt 2016:32–39).

The canons also illuminate the synod’s approach to authority. Authority is neither dissolved into local autonomy nor concentrated in a single juridical centre. Rather, authority is exercised through recognisable offices, received judgements and conciliar procedures. This pattern corresponds to the broader Byzantine understanding of ecclesial order, in which patriarchal dignity, synodal decision-making and imperial context interact in complex ways (Chrysos 2023:155–170; Dvornik 1966:100–118; Polidori 2024:35–42).

Here, the principle of pentarchy becomes relevant. Chrysos’ analysis shows that the councils of 869–870 and 879–880 must be interpreted against the background of the five patriarchal sees and their symbolic ordering within the universal Church (Chrysos 2023:155–170). The canons of 879–880 do not produce a full theory of pentarchy, but they presuppose a world in which ecclesial authority is relational. A judgement made in one centre may require recognition elsewhere; a rupture in one patriarchal relation can threaten wider communion; a restoration must be received through conciliar action.

The canons, therefore, give concrete form to the article’s central thesis. Communion is not merely the invisible unity of believers; it is a visible and ordered reality. It has doctrinal content, a canonical structure and practices of mutual recognition. The Synod of Constantinople 879–880 shows that the Church protects communion not only by confessing the Creed, but also by regulating the conduct of bishops, monks and ecclesiastical authorities. The boundaries of orthodoxy are therefore simultaneously doctrinal and canonical (Gemeinhardt 2016:37–39; Meijer 1975:123–141).

This canonical dimension also prevents a romanticised interpretation of communion. Communion is not sentiment, consensus or spiritual atmosphere. It is a demanding ecclesial reality that requires judgement, discipline and sometimes exclusion. Yet exclusion is not the council’s final aim. The ultimate aim is restoration: the healing of ruptured communion through truth, order and recognition. In this sense, the canons of 879–880 are not merely restrictive. They are therapeutic. They seek to prevent the recurrence of the very disorders that had wounded the Church in the preceding decades.

Thus, the horos and canons must be read together. The horos defines the doctrinal grammar of communion through fidelity to the Creed; the canons define its ecclesial grammar through disciplined order. Together, they reveal the Synod of Constantinople 879–880 as a theological witness to the inseparability of orthodoxy and communion.

The Synod of Constantinople 879–880 cannot be understood without the problem of primacy. The conflict around Photius had become a test case for the relation between the Roman see, the patriarchate of Constantinople and the wider conciliar body of the Church. Yet the synod does not present a simple opposition between papal primacy and Eastern synodality. Rather, it reveals a more complex ecclesial grammar in which primacy, recognition and reception are mutually implicated (Dvornik 1948:216–236; Meijer 1975:123–141; Stephanou 1973:363–407).

The role of Pope John VIII is central. The restoration of communion was not achieved apart from Rome, nor against Rome in any simple sense. The papal legates participated in the council, and the rehabilitation of Photius was framed within a process of renewed ecclesial recognition between Rome and Constantinople (Dvornik 1948:216–236; Meijer 1975:74–122; Polidori 2024:35–42). This fact complicates later polemical readings. The synod was not simply an anti-Roman assembly; it was an attempt to restore communion with Roman participation and within a conciliar framework.

At the same time, the synod does not allow Roman primacy to function as an isolated and self-sufficient juridical mechanism. Its decisions are not presented as the unilateral act of one see but as the judgement of a gathered ecclesial body. The papal legates act within the council, and the council’s authority emerges through synodal reception. This pattern suggests that primacy is real but ecclesially situated; it requires manifestation within communion, not over against communion (Chrysos 2023:155–170; Meijer 1975:123–141).

The principle of pentarchy provides one of the key historical frameworks for this issue. Chrysos shows that the councils of 869–870 and 879–880 must be read in relation to the symbolic and canonical order of the five patriarchal sees: Rome, Constantinople, Alexandria, Antioch and Jerusalem (Chrysos 2023:155–170). This structure did not eliminate Roman primacy, but it embedded primacy within a broader order of patriarchal communion. The unity of the Church was imagined not as the administrative extension of one see, but as a structured communion of apostolic centres.

The Synod of 879–880, therefore, discloses a relational model of authority. Authority is exercised through recognition: Rome recognises Constantinople; Constantinople receives Roman participation; the council recognises Photius; and the wider Church is invited to receive the conciliar judgement. This model is neither congregational nor monarchic. It is conciliar and communional. Ecclesial judgement becomes authoritative not merely because it is pronounced, but because it is received within the ordered communion of the Church (Meijer 1975:123–141; Stephanou 1973:363–407).

This is why reception is not a secondary appendix to conciliar authority. It belongs to the very life of ecclesial judgement. The contrasting receptions of 869–870 and 879–880 demonstrate this point with unusual clarity. The former was later received in the Latin West as the Eighth Ecumenical Council, while the latter gained particular importance in Byzantine and Orthodox memory as the council that restored Photius and defended the Creed against addition (Dragas 1999:357–369; Gemeinhardt 2016:25–39; Stiernon 1975:221–233). Theologically, this divergent reception reveals that conciliar identity is not determined only at the moment of convocation. It is also shaped by the Church’s continuing memory.

The question of reception must therefore be handled carefully. Reception does not mean that truth is created by later consensus. Nor does it mean that a council has no authority until universally accepted. Rather, reception describes the historical and ecclesial process by which a conciliar judgement becomes recognised as belonging to the Church’s normative memory. In this sense, Constantinople 879–880 is an important case because its authority was real, contested and unevenly remembered (Dvornik 1948:216–236; Gemeinhardt 2016:25–39).

The ecclesiological importance of this point is considerable. If a council’s authority is only formal, then the presence of representatives, imperial convocation or canonical procedure would suffice. If authority is only retrospective, then the council would have no intrinsic theological force at the moment of its act. Constantinople 879–880 suggests a more nuanced model: conciliar authority is enacted through formal ecclesial judgement, but its place in the Church’s memory is clarified through reception. The Church judges, and the Church remembers its judgements (Meijer 1975:123–141; Thümmel 2005:437–458).

This has direct relevance for the relation between primacy and synodality. Primacy without reception risks becoming external command; synodality without primacy risks becoming dispersed and unstable. The synod’s practice suggests that ecclesial authority requires both: a recognisable order of sees and a conciliar mode of discernment. Rome has a role; Constantinople has a role; the council has a role; and the whole Church’s reception has a role. Communion is preserved when these dimensions are not severed from one another (Chrysos 2023:155–170; Stephanou 1973:363–407).

The rehabilitation of Photius illustrates this dynamic. His restoration was not merely personal vindication. It became a test of whether previously broken recognition could be healed. In the logic of the synod, the restoration of a patriarch is inseparable from the restoration of communion between churches. A wounded office wounds communion; a healed recognition heals communion. This does not mean that persons and institutions are identical, but it does mean that ecclesial office has a communional significance beyond individual biography (Dvornik 1948:216–236; Meijer 1975:74–122).

The synod’s treatment of the Creed intensifies this point. Primacy and synodality cannot be discussed apart from doctrine. If the Creed is the common boundary of communion, then no primatial or regional authority can unilaterally alter it without affecting the whole Church. The issue is therefore not merely whether the Filioque is theologically defensible, but whether the common symbol of faith can be changed without conciliar reception (Gemeinhardt 2002:41–68; Grumel 1938:357–372; Siecienski 2010:75–108). In this respect, the synod places doctrinal communion above local initiative.

This does not deny the possibility of theological diversity. Latin and Greek theological languages developed different ways of speaking about the procession of the Spirit, and later ecumenical reflection has often distinguished between doctrinal content, linguistic formulation and canonical insertion into the Creed (Chadwick 2003:95–130; Gemeinhardt 2002:41–68; Siecienski 2010:75–108). The point of 879–880, however, is that diversity must remain within the bounds of common confession. The Creed is the shared ecclesial text in which unity becomes audible.

Thus, the Synod of Constantinople 879–880 presents primacy, synodality and reception not as competing principles, but as interdependent dimensions of ecclesial communion. Primacy serves communion when it acts within conciliar recognition. Synodality serves communion when it guards the common faith rather than merely registering political compromise. Reception serves communion when it remembers and transmits what the Church has discerned. In this threefold structure, the boundaries of orthodoxy are defined neither by isolated authority nor by diffuse consensus, but by the Church’s ordered fidelity to the apostolic confession.

Every ecclesiology of communion must also account for exclusion. If communion has form, it also has boundaries. The Synod of Constantinople 879–880 is significant because it does not imagine ecclesial unity as unlimited inclusion. It restores communion, but it does so by reaffirming doctrinal and canonical limits. The horos protects the Creed; the canons regulate ecclesial order; and the council’s acts presuppose that some judgements, depositions and condemnations may either wound or heal communion depending on whether they conform to ecclesial truth (Gemeinhardt 2016:25–39; Meijer 1975:123–141).

The boundary of orthodoxy is, first of all, credal. The Creed is not treated as one theological formulation among others, but as the normative symbol of the Church’s faith. The prohibition against alteration indicates that orthodoxy is not simply a set of ideas but a received form of confession. To stand within communion is to confess the faith in continuity with the Church’s conciliar memory (Gemeinhardt 2002:41–68; Grumel 1938:357–372, 1947:218–234).

The boundary is also canonical. A bishop, monk or cleric does not belong to the Church merely as an isolated believer, but within a disciplined order. The canons of 879–880 show that ecclesial disorder is not external to theology. Arbitrary deposition, ambition for office and irregular recognition all threaten communion because they disrupt the visible order through which the Church’s sacramental and doctrinal life is maintained (Gemeinhardt 2016:37–39; Meijer 1975:123–141).

Yet the synod’s logic of exclusion is not simply punitive. Its deeper movement is restorative. The restoration of Photius shows that the Church’s boundaries are not only instruments of separation, but also conditions for reconciliation. Communion can be restored when the conditions of truth, recognition and order are re-established. In this respect, boundary-making and healing are not opposed. The Church marks boundaries precisely so that communion may be real rather than rhetorical (Dvornik 1948:216–236; Stephanou 1973:363–407).

This point has contemporary theological significance. Modern ecclesial discourse often oscillates between two extremes: a juridical model in which boundaries are reduced to institutional obedience, and a relativising model in which communion is imagined without doctrinal or canonical form. Constantinople 879–880 offers neither model. It presents communion as generous but not formless, ordered but not merely juridical, doctrinally defined but not abstracted from reconciliation (Chrysos 2023:155–170; Meijer 1975:123–141).

The synod also shows that the boundaries of orthodoxy are ecclesial rather than merely intellectual. Orthodoxy is not only a correct opinion. It is a right confession within the body of the Church. This means that doctrine, liturgy, episcopal order and mutual recognition belong together. A breach in one dimension affects the others. The addition to the Creed, the rupture between Rome and Constantinople, the disputed status of Photius and the question of canonical judgement are not separate problems. They are different expressions of one crisis of communion (Gemeinhardt 2016:25–39; Polidori 2024:35–42).

For this reason, the horos and canons should not be read as two unrelated textual units. The horos defines the doctrinal boundary of communion; the canons define its disciplinary boundary. Together, they reveal a Church that understands itself as a communion of faith and order. The problem of orthodoxy is therefore inseparable from the problem of ecclesial visibility. The Church must be able to say what faith it confesses, whom it recognises and how it heals ruptures in its own body.

In this sense, Constantinople 879–880 may be interpreted as a conciliar act of ecclesial discernment. It discerns the true form of communion after decades of conflict. It distinguishes between legitimate and illegitimate rupture, between inherited confession and unauthorised alteration, between primacy exercised within communion and authority asserted apart from reception. Its boundaries are therefore not merely negative. They are constructive forms by which the Church remains recognisably itself.

The conclusion follows: the Synod of Constantinople 879–880 is a witness to an ecclesiology in which communion and orthodoxy are mutually constitutive. Communion without orthodoxy becomes indeterminate; orthodoxy without communion becomes abstraction or polemic. The synod’s theological significance lies precisely in holding these together.

The preceding analysis allows the Synod of Constantinople 879–880 to be interpreted as a locus theologicus of ecclesial communion. This does not mean that the synod should be abstracted from its historical contingencies or converted into a timeless model without remainder. Rather, it means that the conciliar act itself becomes a theological place in which the Church’s understanding of communion, orthodoxy and authority is rendered visible (Gemeinhardt 2016:25–39; Meijer 1975:123–141).

In classical theological method, a locus theologicus is not merely a topic but a source or place from which theological reasoning may responsibly proceed. The Synod of 879–880 functions in precisely this sense because it gathers together several constitutive dimensions of ecclesial life: confession of faith, canonical order, patriarchal recognition, conciliar judgement and reception. Its theological importance lies not in one isolated doctrinal innovation, but in the way these dimensions are integrated into a single ecclesial act (Meijer 1975:74–122; Stephanou 1973:363–407).

The first dimension is doctrinal. The horos witnesses to a Church whose unity depends upon fidelity to the inherited Creed. The Creed is not simply one doctrinal formulation among many; it is the normative symbol by which communion becomes confessable and recognisable. In this respect, Constantinople 879–880 shows that the unity of the Church cannot be severed from the integrity of the Church’s confession (Gemeinhardt 2002:41–68; Grumel 1938:357–372).

The second dimension is canonical. The canons show that communion requires order, discipline and recognisable ecclesiastical procedures. The Church’s unity is not an invisible ideal floating above history, but a visible and structured reality embodied in bishops, patriarchates, synods and canonical judgements. Canon law here functions not as an external administrative mechanism but as the disciplined form of communion (Gemeinhardt 2016:37–39; Meijer 1975:123–141).

The third dimension is reconciliatory. The synod is concerned not only with defining error but with restoring communion. This is essential. Constantinople 879–880 does not simply condemn; it heals. It seeks to bring an ecclesial body wounded by deposition, mutual suspicion and competing recognitions back into a state of peace. The rehabilitation of Photius is therefore not merely personal or institutional. It is a theological sign of restored ecclesial recognisability (Dvornik 1948:216–236; Polidori 2024:35–42).

The fourth dimension is conciliar. The synod shows that ecclesial judgement is not reducible to the decision of one office, even when primacy is acknowledged. Authority is exercised through a gathered body, through recognition by the participating sees and through the later reception of the Church. This conciliar form does not negate primacy; rather, it places primacy within the wider grammar of communion (Chrysos 2023:155–170; Stephanou 1973:363–407).

For these reasons, Constantinople 879–880 may be read as a theological source for contemporary ecclesiology. It clarifies that communion is not a sentimental category, not a merely juridical status and not an abstract doctrinal agreement. Communion is the visible life of the Church as constituted by shared faith, recognised order and reconciled ecclesial memory. This makes the synod especially relevant for Orthodox–Catholic dialogue, where the questions of Creed, primacy, synodality and reception remain inseparable (Chadwick 2003:95–130; Chrysos 2023:155–170; Siecienski 2010:75–108).

The synod’s value is therefore not exhausted by the question of whether it should be numbered among the ecumenical councils. That question is historically and confessionally important, but it can also obscure the broader theological significance of the event. Even where its ecumenical status remains disputed, the synod remains a major witness to how the ninth-century Church imagined the restoration of communion after rupture. It is precisely this witness that gives it enduring theological relevance (Dvornik 1948:216–236; Gemeinhardt 2016:25–39; Meijer 1975:123–141).

The Synod of Constantinople 879–880 speaks to contemporary theology in at least four significant ways. Firstly, it challenges any account of ecclesial communion that separates unity from confession. In modern ecumenical discourse, communion is sometimes treated primarily as relational openness or institutional rapprochement. The synod suggests a more demanding model: communion is possible only where the common confession of faith is preserved and recognised (Gemeinhardt 2002:41–68; Grumel 1938:357–372).

Secondly, the synod challenges any account of orthodoxy that separates doctrinal correctness from ecclesial reconciliation. The council does not defend the Creed in order to perpetuate division, but in order to restore the conditions of communion. Orthodoxy is therefore not a weapon against communion; it is the form that makes true communion possible. This distinction is crucial for contemporary discussions in which doctrinal boundaries are often perceived as obstacles to unity rather than as conditions of truthful unity (Meijer 1975:123–141; Stephanou 1973:363–407).

Thirdly, the synod offers a historically grounded model for thinking about primacy and synodality together. It neither abolishes the role of Rome nor grants Roman primacy an isolated function detached from conciliar reception. The participation of Roman legates and the conciliar rehabilitation of Photius show a model in which primacy acts within communion and synodality receives and tests ecclesial judgement (Chrysos 2023:155–170; Dvornik 1948:216–236).

Fourthly, the synod demonstrates that reception is not accidental to ecclesiology. The divergent later receptions of 869–870 and 879–880 reveal that the authority of councils is historically mediated through memory, recognition and ecclesial use. This does not relativise truth, but it does show that the Church’s normative memory is formed through processes of reception that must themselves be studied theologically (Gemeinhardt 2016:25–39; Stiernon 1975:221–233; Thümmel 2005:437–458).

The contemporary relevance of Constantinople 879–880 is therefore not limited to specialists in Byzantine history. The synod raises questions that remain central: Can the Creed be treated as a shared rule of communion rather than as a confessional possession? Can primacy be exercised in a way that is neither unilateral nor merely symbolic? Can synodality protect doctrinal identity without degenerating into political compromise? Can ecclesial boundaries be understood as conditions of healing rather than as instruments of exclusion?

From the perspective of Orthodox–Catholic dialogue, the synod offers both a challenge and a resource. It challenges Rome to reckon with the historical limits of unilateral alteration of a shared Creed and with the ecclesiological implications of reception. It challenges Orthodoxy to articulate the positive form of primacy within communion rather than merely opposing later Latin developments. It challenges both traditions to recognise that communion cannot be reconstructed without a shared account of faith, order and memory (Chadwick 2003:95–130; Chrysos 2023:155–170; Siecienski 2010:75–108).

In this sense, Constantinople 879–880 should not be used merely as a polemical weapon in confessional apologetics. Its deeper significance lies elsewhere: it shows the Church attempting to heal a rupture by returning to the shared Creed, restoring mutual recognition and disciplining ecclesial order. This is precisely why the synod remains a theological source for the present. It does not solve all later problems, but it discloses a grammar by which those problems may be more responsibly addressed.

This article has argued that the Synod of Constantinople 879–880 should be interpreted as a significant witness to an ecclesiology of communion. Against readings that reduce the synod either to an episode in the Photian conflict or to a merely polemical moment in the history of the Filioque controversy, the study has proposed that its horos and canons articulate a coherent theological vision of the Church. In this vision, communion is constituted by fidelity to the inherited Creed, canonical order, mutual recognition and conciliar restoration.

The first conclusion concerns the horos. The synod’s defence of the Nicene-Constantinopolitan Creed reveals that doctrinal confession functions as the boundary of ecclesial communion. The prohibition of unauthorised additions is not merely textual conservatism. It is an ecclesiological claim: the common symbol of faith belongs to the whole Church and cannot be altered by unilateral action (Gemeinhardt 2002:41–68; Grumel 1938:357–372).

The second conclusion concerns the canons. The disciplinary material of 879–880 shows that communion requires canonical form. The Church’s unity is not preserved by doctrine alone, considered abstractly, but by an ordered ecclesial life in which bishops, monks, patriarchates and synods are bound by recognisable norms. Canonical order is therefore not external to theology; it is one of the visible forms in which communion is protected (Gemeinhardt 2016:37–39; Meijer 1975:123–141).

The third conclusion concerns primacy and synodality. The synod shows that primacy and synodality need not be construed as mutually exclusive principles. Roman participation in the restoration of communion is real, but it is exercised within a conciliar process. The authority of the synod emerges through gathered judgement, patriarchal recognition and later reception. This suggests a communional model of authority in which primacy serves unity precisely by remaining accountable to conciliar discernment (Chrysos 2023:155–170; Dvornik 1948:216–236; Stephanou 1973:363–407).

The fourth conclusion concerns the boundaries of orthodoxy. Constantinople 879–880 shows that boundaries are not merely negative instruments of exclusion. They are also constructive forms that make reconciliation possible. By defining the Creed, regulating ecclesial order and restoring Photius, the synod marks the conditions under which communion can be healed. Orthodoxy and communion are therefore mutually constitutive: communion without orthodoxy becomes formless, while orthodoxy without communion becomes abstraction or polemic.

The article has therefore treated the Synod of Constantinople 879–880 as a locus theologicus: a historically situated source in which the Church’s theological self-understanding becomes visible. Its significance does not depend solely on whether one receives it as ecumenical in the formal sense. Even before that question is answered, the synod remains a major theological witness to how the Church negotiates unity, authority, memory and doctrinal fidelity after rupture.

For contemporary ecclesiology, this witness is especially valuable. In an age marked by renewed debates over synodality, primacy, reception and the limits of communion, Constantinople 879–880 offers neither a simple solution nor a nostalgic model. It offers something more useful: a conciliar grammar. That grammar teaches that the Church remains one by confessing the same faith, receiving legitimate judgement, preserving canonical order and healing ruptures through truth. This is the enduring theological relevance of the synod.