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One of the major reasons behind this power struggle in the northern area of the Byzantine world was the problem of relations with the Western Church. To most Byzantine churchmen, the young Muscovite principality appeared to be a safer bulwark of Orthodoxy than the Western-oriented princes who had submitted to Catholic Poland and Lithuania. Also, an important political party in Byzantium itself favoured union with the West in the hope that a new Western Crusade might be made against the menacing Turks. The problem of ecclesiastical union was, in fact, the most burning issue during the entire Palaeologan period.
Emperor Michael Palaeologus (1259-82) had to face the aggressive ambition of the Sicilian Norman king Charles of Anjou, who dreamed of restoring the Latin empire in Constantinople. To gain the valuable support of the papacy against Charles, Michael sent a Latin-inspired confession of faith to Pope Gregory X, and his delegates accepted union with Rome at the Council of Lyons (1274). This capitulation before the West, sponsored by the Emperor, won little support in the church. During his lifetime, Michael succeeded in imposing an Eastern Catholic patriarch, John Beccus, upon the Church of Constantinople, but upon Michael's death an Orthodox council condemned the union (1285).
Throughout the 14th century, numerous other attempts at negotiating union were initiated by the emperors of Byzantium. Formal meetings were held in 1333, 1339, 1347, and 1355. In 1369 Emperor John V Palaeologus was personally converted to the Roman faith in Rome. All these attempts were initiated by the government and not by the church, for an obvious political reason; i.e., the hope for Western help against the Turks. But the attempts brought no results either on the ecclesiastical or on the political levels. The majority of Byzantine Orthodox churchmen were not opposed to the idea of union but considered that it could only be brought about through a formal ecumenical council at which East and West would meet on equal footing, as they had done in the early centuries of the church. The project of a council was promoted with particular consistency by John Cantacuzenus, who, after a brief reign as emperor (1347-54), became a monk but continued to exercise great influence on all ecclesiastical and political events. The idea of an ecumenical council was initially rejected by the popes, but it was revived in the 15th century with the temporary triumph of conciliarist ideas (which advocated more power to councils and less to popes) in the West at the councils of Constance and Basel. Challenged with the possibility that the Greeks would unite with the conciliarists and not with Rome, Pope Eugenius IV called an ecumenical council of union in Ferrara, which later moved to Florence.
The Council of Ferrara-Florence (1438-45) lasted for months and allowed for long theological debates. Emperor John VIII Palaeologus, Patriarch Joseph, and numerous bishops and theologians represented the Eastern Church. They finally accepted most Roman positions—the Filioque clause, purgatory (an intermediate stage for the soul's purification between death and heaven), and the Roman primacy. Political desperation and the fear of facing the Turks again, without Western support, was the decisive factor that caused them to place their signatures of approval on the Decree of Union (July 6, 1439). The metropolitan of Ephesus, Mark Eugenicus, alone refused to sign. Upon their return to Constantinople, most other delegates also renounced their acceptance of the council and no significant change occurred in the relations between the churches.
The official proclamation of the union in Hagia Sophia was postponed until December 12, 1452; however, on May 29, 1453, Constantinople fell to the Ottoman Turks. Sultan Mehmed II transformed Hagia Sophia into an Islamic mosque, and the few partisans of the union fled to Italy.