In 1438 the Christian East came west in force. The Emperor John VIII, the aged Patriarch Joseph II, and some seven hundred Greeks crossed to Ferrara and then Florence, seeking two things at once: the healing of the schism, and military aid against the Ottoman armies already closing on a dying Constantinople. For more than a year they debated the Latins as equals on the Filioque, purgatory, the bread of the Eucharist, and the papacy.

The fruit was the Bull of Union, Laetentur Caeli, signed on 6 July 1439 by nearly the whole Greek delegation — and it was no crude Latin victory. It defined the Spirit’s procession “from one principle,” not two causes; it explicitly equated the Greek “through the Son” with the Latin “from the Son”; it ruled that the Body of Christ is “truly confected in both unleavened and leavened” bread, vindicating both rites; and it issued the first conciliar definition of the papal primacy — qualified, tellingly, “as is contained also in the acts of the ecumenical councils and in the sacred canons.”

One bishop would not sign. Mark Eugenikos, Metropolitan of Ephesus, stood alone; when the Pope learned of it he is said to have grasped at once what it meant — “Then we have accomplished nothing” (the line comes through the anti-unionist Syropoulos, so take it as tradition, not minutes). Mark was right about the outcome. The delegation returned to a Church that recoiled: bishops who had signed recanted, the faithful refused communion with the unionists, and in 1484 a synod of all four Eastern patriarchates formally repudiated the union. Mark is venerated as a Pillar of Orthodoxy — the conscience the rest had overruled.

The Catholic claim is precise, and worth stating exactly: Florence was a true ecumenical council with both lungs of the Church present, and the doctrine it defined was sound. Its tragedy was political collapse and non-reception, not error — the promised aid never came, Constantinople fell to the Turks in 1453, and a union signed by exhausted, stranded, grieving men never took root. But here honesty is required, because this is the Orthodox case at its strongest. The duress was real, and the non-reception was nearly total — and since the Orthodox test for a true council is precisely reception by the whole Church, Florence cannot simply be invoked as “the council that settled it.” It won the argument on paper and lost the Church in the East. That is the honest shape of it.