Council of Florence participant Isidore of Kyiv
Meeting in Florence
The Council had meanwhile successfully negotiated reunification with several Eastern Churches, reaching agreements on such matters as the Western insertion of the phrase "Filioque" to the Nicene-Constantinopolitan Creed, the definition and number of the sacraments, and the doctrine of Purgatory. Another key issue was papal primacy, which involved the universal and supreme jurisdiction of the Bishop of Rome over the whole Church, including the national Churches of the East (Serbian, Byzantine, Moldo-Wallachian, Bulgarian, Russian, Georgian, Armenian etc.), and nonreligious matters such as the promise of military assistance against the Ottoman Empire.
On 6 July 1439 the union was proclaimed (in both Latin and Greek) in the document Laetentur Caeli ("Let the Heavens Rejoice") which was signed by Pope Eugene and by the Byzantine Emperor John VIII Palaiologos. All but one of the bishops were present. Some Greek bishops, perhaps feeling political pressure from the Byzantine Emperor, reluctantly accepted the decrees of the Council. Other Eastern bishops did so by sincere conviction, such as Isidore of Kiev, who subsequently suffered greatly for it. Only one Eastern Bishop refused to accept the union, Mark of Ephesus, who became the leader of opposition back in Byzantium; the Serbian patriarch did not even attend the council. The Russian Orthodox Church, upon learning of the union, angrily rejected it and ousted any prelate who was even remotely sympathetic to it, declaring itself autocephalous (i.e., autonomous).
Despite the religious union, Western military assistance to Byzantium was ultimately insufficient, and the fall of Constantinople occurred in May 1453. The Council declared the Basel group heretics and excommunicated them, and affirmed the superiority of the Pope over the Councils in the bull Etsi non dubitemus of 20 April 1441.