Isidore of Kiev, also known as Isidore of Thessalonica or Isidore, the Apostate (; ; ; 1385 – 27 April 1463), was a prelate of Byzantine Greek origin. From 1437 to 1441 he served as the Metropolitan of Kiev and all Rus' in the patriarchate of Constantinople of the Eastern Orthodox Church. He was a supporter of the Union of Florence which he proclaimed in Hagia Sophia on 12 December 1452. In the Latin Church, Isidore was the cardinal bishop of Sabina, Archbishop of Cyprus, Camerlengo of the Sacred College of Cardinals and the Latin Patriarch of Constantinople. Early life Isidore was born in southern Greece circa 1385. He arrived at Constantinople, became a monk, and was there made hegumenos of the monastery of St Demetrius. He knew Latin well, and had considerable fame as a theologian. He was also an accomplished orator; he seems from the beginning to have been eager for reunion with the West. It was the time when the Court of Constantinople, on the eve of its final destruction by the Turks, was considering the chance of rescue from the Western princes as a result of reuniting with Rome. In 1434 Isidore was sent to Basel by John VIII Palaiologos (1425–1448) as part of an embassy to open negotiations with the Council of Basel. Here he made a mellifluous speech about the splendour of the Roman Empire at Constantinople, but his efforts did not result in union of the churches. Metropolitan of Kiev In 1437, Isidore was appointed Metropolitan of Kiev and all Rus' by Emperor John VIII Palaeologus and consecrated by Patriarch Joseph II. The Emperor hoped to draw the Eastern Orthodox Church in Kievan Rus' into communion with the Holy See and thereby to secure Constantinople's protection against the invading Ottoman Turks. The Grand Duke of Moscow — Vasili II — was suspicious of the new metropolitan. He allowed Isidore to go to Florence to attend the continuation of the Council of Basel in 1439 on condition that Isidore should return with "the rights of Divine law and the constitution of the holy Church" uninjured. Isidore assured him that he would return from the ecumenical council without betraying the Orthodox faith. During this Council, Isidore fervently defended the union between the Churches of East and West, but he was opposed by the secular representative from Ruthenia - ambassador Foma (Thomas) of Tver. Finally, the