The eastern and western churches split over differences in theology, practice, politics and culture. As Constantinople became an important center of government in the eastern half of the Roman Empire, the church there began to challenge the supremacy of Rome. Without a common language and culture, the two halves of the Christian Church drifted apart in the face of their disagreements.
The split between the eastern and western churches was largely politically motivated. The Bishop of Rome held considerable authority from the early days of the church. Rome, Alexandria and Antioch were the three power centers of Christianity. Once Constantine moved the capital of the Roman Empire to Byzantium, the church there enjoyed increased ecclesiastical influence equal to the city's bolstered political importance. After Alexandria and Antioch fell to the Muslims, Rome and Constantinople remained the two strongest churches, creating a rivalry that fueled their eventual separation.
The churches of the east and west disagreed on key points of doctrine and ritual. The western church believed that the Holy Ghost emanates from the Son as well as the Father. The clergy of the western church angered the church in Constantinople when it inserted this doctrine into the Nicene creed. The western church adopted the use of unleavened bread for the Eucharist to the chagrin of the eastern church. The eastern church disagreed with the western tradition of unmarried priests. The Roman church disdained the eastern church's subservience to the Byzantine Emperor.
These tensions came to a climax when Patriarch of Constantinople Michael Cerularius ordered the closure of all Latin churches in his city. When the leadership of the western church excommunicated him, Cerularius excommunicated them in turn.