Florence Revisited: Patristic Unity, Doctrinal Integrity, and the Crisis Raised by In Unitate Fidei
A Moment Demanding Historical Memory
On 23 November 2025, Pope Leo XIV issued the apostolic letter In Unitate Fidei to commemorate the 1700th anniversary of the Council of Nicaea. In it, he offered the Creed in its 381 Nicene–Constantinopolitan form—omitting the Filioque—and remarked that certain “theological controversies have lost their raison d’être.”¹ This gesture, framed as an invitation to Christian unity, has nevertheless placed the Church before a profound doctrinal question. For the Filioque, treated for centuries by the Latin West as de fide and reaffirmed solemnly at the Council of Florence, cannot simply be set aside without revisiting the entire theological architecture upon which East–West doctrine has been understood.
This moment therefore compels the Church not to reduce doctrine for the sake of unity, but to return to the last time unity was achieved without compromise: the Council of Florence (1438–1445), where East and West found themselves in agreement by appealing not to modern diplomacy but to the Fathers.
The World in Which Florence Emerged
By the 1430s, the Byzantine Empire, once the bastion of Christian civilisation, was nearing its final hour. Constantinople was an exhausted sentinel silhouetted against the encircling shadow of Ottoman arms. In this twilight, Emperor John VIII Palaiologos recognised that neither armies nor treaties would save the empire. Only unity—unity grounded in truth—could give Christendom a fighting chance.
Thus the emperor undertook an act that astonished observers across Europe: he travelled personally into the Latin West. He came not in triumph but in humility, accompanied by the Patriarch of Constantinople, his metropolitans, his theologians, and a retinue of scholars and monks.² His purpose was not merely political; it was ecclesial, even penitential. He sought not advantage but reconciliation, not favour but fidelity to the faith of the first millennium.
The Rediscovery of the Fathers
The ensuing council—first in Ferrara, then in Florence—became a vast, living patristic library. Never since the early centuries had East and West so intently opened the books of the Fathers together. Day after day the voices of Basil, Gregory of Nyssa, Gregory Nazianzen, Athanasius, Cyril of Alexandria, Augustine, and Maximus the Confessor filled the chamber.³ Manuscripts were compared, Greek and Latin were weighed in careful balance, and centuries of polemic were set aside so that the Fathers could speak with clarity.
The very air of Florence seemed suffused with the memory of the undivided Church. Arguments that had calcified into impasses suddenly yielded when placed beneath the glow of patristic consensus. For the first time in generations, East and West could say not merely “we disagree less,” but “we have found in the Fathers the same truth.”
The Filioque Resolved in Cyril’s Light
The procession of the Holy Spirit—so long a cause of rupture—found its resolution in the writings of St Cyril of Alexandria. Cyril insisted that the Spirit proceeds essentially from the Father yet is communicated eternally through the Son.⁴ This, the Greeks realised, was not a compromise with the Latin formula but the very patristic key to interpreting it. Cyril’s voice dissolved the fog of polemic. In his theology, both sides discovered that they had been defending the same mystery in different accents.
Petrine Primacy Rediscovered Through Maximus
Similarly decisive was the testimony of St Maximus the Confessor. Maximus, revered across the East, had declared that Rome “presides in truth” because it safeguards the apostolic faith without deviation.⁵ The Greeks could not dismiss this saintly voice. They recognised in Maximus the ancient and authentic witness to the Roman primacy—the primacy not of domination but of doctrinal guardianship. This primacy belonged not to medieval politics but to the first millennium, the millennium both Churches revered.
Purgatory, the Eucharist, and the Shared Sacramental Tradition
Doctrines often caricatured as Western inventions were shown to be deeply rooted in the common patristic inheritance. St Gregory of Nyssa’s meditations on purification after death, Chrysostom’s insistence on praying for the departed, Augustine’s commentary on 1 Corinthians 3, and Gregory the Great’s Dialogues revealed that the Latin teaching on purgatory was nothing other than the ancient doctrine of sanctification carried beyond death.⁶
Likewise, conflicts over Eucharistic matter dissolved when early sources—Cyril of Jerusalem, Ignatius of Antioch, Irenaeus—confirmed that the validity of the Sacrament rests not on leaven but on institution and consecration.⁷
The Fathers smoothed every path.
The Union Signed in Tears
When Laetentur Caeli was signed on 6 July 1439, bishops from both sides wept. They recognised that the division between East and West had not been doctrinal at its heart, but historical—born of miscommunications, political tragedies, and hardened suspicions. Joseph Gill recounts Eastern bishops embracing Latin theologians, thanking them for revealing in the Fathers the forgotten unity of their heritage.⁸ Syropoulos himself, later critical of the union, admitted it had been forged “through constant appeals to the holy patristic writings.”⁹
Unity had been restored—not by compromise, but by truth.
The Fall of Constantinople and the Collapse of the Union
Within fourteen years, however, the walls of Constantinople collapsed beneath the Ottoman assault. With the empire fell the political and ecclesial structures needed to sustain the union. Under Islamic rule, suspicion of Latin loyalties became a survival strategy, and bishops loyal to Florence found themselves replaced.¹⁰ The union did not fail because its theology was flawed. It failed because the empire that signed it ceased to exist.
Florence was not a failure. It was a victory cut short by history.
A Modern Echo: Archbishop Mathew and the Eastern Patriarchates
Centuries later, the same principle resurfaced. In 1912, Archbishop Arnold Harris Mathew, founder of the Old Roman Apostolate, received fraternal recognition from the Patriarchates of Antioch and Jerusalem. These Eastern Churches, evaluating the lineage and doctrine of the Old Roman clergy, discerned in them the continuity of the ancient Catholic and apostolic faith—even after centuries of separation.¹¹
This episode echoed Florence: unity, when grounded in the Fathers, is natural; obstruction arises only from politics and fear.
The Challenge of In Unitate Fidei
Against this backdrop, Pope Leo XIV’s apostolic letter raises urgent questions. By returning liturgically to the 381 Creed and deeming the Filioque controversy exhausted, the Pope signals a pastoral desire for unity. But doctrinally, such a move reopens issues Florence had settled carefully and authoritatively. The Filioque was not an appendix added for polemical convenience; it was the fruit of exhaustive patristic study, affirmed solemnly by an ecumenical council.
If the Filioque can be deemed to have “lost its raison d’être,” what becomes of Florence’s dogmatic authority? What becomes of the principle that doctrine, once defined, is not negotiable? What becomes of Rome’s claim to doctrinal guardianship if that guardianship can shift in the face of ecumenical diplomacy?
These questions do not arise from suspicion but from fidelity. To revisit the Creed is to revisit Florence. And to revisit Florence is to confront the binding claims of its doctrine.
Why the Church Must Return to Florence Now
Florence is not antiquarian memory but living obligation. It remains the last moment in history when East and West achieved full doctrinal unity without compromise. Modern ecumenism tends toward reduction: unity through subtraction, agreement by silence. Florence shows a better way: unity through fullness, agreement through truth.
The Church stands now at a crossroad. She may choose a unity founded on selective memory, trimming doctrines that others find uncomfortable. Or she may return to the path illuminated at Florence and rediscovered in the recognitions of 1912: unity grounded in the Fathers, the Councils, and the entirety of the apostolic faith.
If unity is truly desired today, Florence shows the way. Not a reduction, but a restoration. Not diplomacy, but doctrine. Not flexibility, but fidelity.
Christ’s prayer “that they all may be one” was fulfilled once already. It remains only for the Church to desire unity deeply enough to allow the Fathers to speak again.
- Pope Leo XIV, In Unitate Fidei (23 Nov 2025).
- Joseph Gill, The Council of Florence (Cambridge University Press, 1959), 74–102.
- Ibid., 145.
- St Cyril of Alexandria, Thesaurus, PG 75:585–588; Gill, 286–298.
- St Maximus the Confessor, Opuscula Theologica et Polemica, PG 91:137–140.
- St Gregory of Nyssa, On the Soul and the Resurrection; St Augustine, Enchiridion; St Gregory the Great, Dialogues.
- St Cyril of Jerusalem, Mystagogical Catecheses; St Irenaeus, Against Heresies.
- Gill, 358–365.
- Sylvester Syropoulos, Memoirs, ed. V. Laurent (Paris: CNRS, 1971).
- Steven Runciman, The Fall of Constantinople 1453 (Cambridge University Press, 1965).
- A. H. Mathew, Letters and Documents (1912); Peter-Ben Smit, “Old Catholic and Old Roman Relations,” Utrecht Studies, 2014.
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