History and schism
The Great Schism
The doctrine of the Christian Church was established over the centuries at Councils dating from as early as 325CE where the leaders from all the Christian communities were represented. The Eastern Church recognizes the authority of the Councils of Nicea 325 CE, Constantinople I (381), Ephesus (431) Chalcedon (451) Constantinople II (553), Constantinople III (680) and Nicaea II (787).
Although initially the Eastern and Western Christians shared the same faith, the two traditions began to divide after the seventh Ecumenical Council in 787 CE and is commonly believed to have finally split over the conflict with Rome in the so called Great Schism in 1054.
In particular this happened over the papal claim to supreme authority and the doctrine of the Holy Spirit. The break became final with the failure of the Council of Florence in the fifteenth century.
However, in the minds of most Orthodox, a decisive moment was the sack of Constantinople in 1204 during the (Western Christian) Fourth Crusade. The sacking of Constantinople by the Crusaders eventually led to the loss of this Byzantine capital to the Muslim Ottomans in 1453. This has never been forgotten.
The divisions between the East and Western Churches happened gradually over the centuries as the Roman Empire fragmented.
Eventually, while the Eastern Churches maintained the principle that the Church should keep to the local language of the community, Latin became the language of the Western Church.
Until the schism the five great patriarchal sees were Rome, Constantinople, Alexandria, Antioch and Jerusalem. After the break with Rome Orthodoxy became 'Eastern' and the dominant expression of Christianity in the eastern Mediterranean, much of Asia Minor, Russian and Balkans.