In 1438 Pope Eugenius IV called a church council to consider reunion of the eastern and western churches. The Latin and Greek churches had been drifting apart for centuries and from the year 1054 onward had rarely been in communion with each other. The sack of the Byzantine capital of Constantinople by the western crusaders made it clear that they no longer considered the Greeks their coreligionists and proved to the Greeks of Byzantium that the Latins were not their brothers in faith. But by the fifteenth century, with the Ottoman Turks already in control of most of the territory of the Byzantine Empire and moving on its capital of Constantinople, reunion of the churches seemed to be a necessity if the Christian world were to respond with a united front to the Muslim threat to Europe.
The council convened in 1439 in the Italian city of Ferrara and then moved to Florence. Present were not only the Pope, the cardinals, and many western bishops and theologians, but also the Byzantine Emperor John VIII, the Patriarch of Constantinople, Joseph II, the foremost cleric of the eastern Christian world, and a number of leading officials and clergy of the Byzantine world (including a Russian delegation). The main points of dispute between the two churches were the legitimacy of a western addition to the creed (the "filioque") and the nature of the church: whether it should be ruled by the Pope or by all the bishops jointly. After much discussion and debate, the delegates of the eastern church, under political pressure, accepted the western positions on the "filioque" and Papal supremacy, and reunion of the churches was solemnly proclaimed.Τhat is almost proclaimed.Mark bishop of Ephesus was thus the only Eastern bishop to refuse to sign the decrees of the council.He is famous for his defense of Eastern Orthodoxy at the Council of Florence in spite of Byzantine Emperor Ioannis VIII Palaeologus and Pope Eugene IV.For his efforts at the Council of Florence and his defense against the addition of the Filioque, the Orthodox Church considers him a saint, calling him a Pillar of Orthodoxy. His feast day is January 19.
When the Greek representatives returned home, however, their decision was greeted with derision. Church union was never accepted by the masses of the Eastern Christian faithful. In any case, it became a dead letter with the 1453 Turkish conquest of Constantinople, renamed Istanbul by the Turks. When the Greek Isidore, Metropolitan of Kiev and presiding bishop of the Russian church, returned to Moscow where he normally resided and proclaimed the Pope as the head of the church, he was arrested on the orders of Grand Prince Basil II ("The Dark") and then diplomatically allowed to escape to Poland. In 1448 he was replaced as metropolitan by a Russian bishop, Jonah, without the consent of the mother church in Constantinople, which was deemed to have given up its faith by submitting to the Pope. From now on, the church of Russia would be an independent (autocephalous) Orthodox church.
The ramifications of the Council of Florence were significant. The rejection of its decisions in the East made it clear that the Roman Catholic and Orthodox churches were to be separate institutions, as they are today. Yet the concept of incorporating eastern ritual into Catholicism in certain places, a compromise that evolved at the council, became the model for the so-called uniate church created in Polish-governed Ukraine and Belarus in 1596, whereby the Orthodox church in those lands became part of the Catholic church while retaining its traditional eastern rites.