Historically, the contemporary Orthodox church stands in direct continuity with the earliest Christian communities founded in regions of the Eastern Mediterranean by the apostles of Jesus. The subsequent destinies of Christianity in those areas were shaped by the transfer (320) of the imperial capital from Rome to Constantinople by Constantine I.
As a consequence, during the first 8 centuries of Christian history most major intellectual, cultural, and social developments in the Christian church also took place in that region; for example, all ecumenical councils of that period met either in Constantinople or in its vicinity. Missionaries, coming from Constantinople, converted the Slavs and other peoples of Eastern Europe to Christianity (Bulgaria, 864; Russia, 988) and translated Scripture and liturgical texts into the vernacular languages used in the various regions. Thus, the liturgy, traditions, and practices of the church of Constantinople were adopted by all and still provide the basic patterns and ethos of contemporary Orthodoxy.
These developments, however, were not always consistent with the evolution of Western Christianity, where the bishop of Rome, or pope, came to be considered the successor of the apostle Peter and head of the universal church by divine appointment. Eastern Christians were willing to accept the pope only as first among patriarchs. This difference in approach explains the various incidents that grew into a serious estrangement. One of the most vehement disputes concerned the filioque clause of the Nicene Creed, which the Western church added unilaterally to the original text.
The schism developed gradually. The first major breach came in the 9th century when the pope refused to recognize the election of PHOTIUS as patriarch of Constantinople. Photius in turn challenged the right of the papacy to rule on the matter and denounced the filioque clause as a Western innovation. The mounting disputes between East and West reached another climax in 1054, when mutual anathemas were exchanged. The sacking of Constantinople by the Fourth Crusade (1204) intensified Eastern hostility toward the West. Attempts at reconciliation at the councils of Lyon (1274) and Florence (1438-39) were unsuccessful. When the papacy defined itself as infallible (First VATICAN COUNCIL, 1870), the gulf between East and West grew wider. Only since the Second VATICAN COUNCIL (1962-65) has the movement reversed, bringing serious attempts at mutual understanding.
Adapted from: John Meyendorff. The Orthodox Church. The Academic American Encyclopedia (Electronic Version), copyright © 1993 Grolier, Inc., Danbury, CT.