The schism between the Byzantine Church in Constantinople and the Church of Rome developed gradually from causes that were theological, cultural, and political. The use of the title Ecumenical Patriarch of the Patriarch of Constantinople, the coronation of Charlemagne by Pope Leo III as emperor of the Holy Roman Empire in opposition to the Roman Emperor of Byzantium, the dispute over the use of leavened or unleavened bread in the Eucharist, debates over clerical celibacy, the papal excommunication of the Orthodox in 1054, and finally the sack of Constantinople by the Crusaders in 1204 and the subsequent installation of a Latin patriarch in the Great Church, Hagia Sophia, all exemplify the deteriorating relationship between the Orthodox and Western churches. Yet the Orthodox perceive the fundamental issues underlying the lasting schism to be chiefly two: the development of the claim for papal primacy over all bishops including those in the East, and the Western introduction of the filioque, the claim that the Holy Spirit proceeds from the Father and the Son, into the Nicene Creed. Political expedience led emperors to seek union at the Council of Lyons (1274) and Council of Florence (1438-39), but these unions were renounced by the Orthodox Church.