Almost from the very beginning, Christians referred to the Church as the “One, Holy, Catholic (from the Greek καθολική, or “according to the whole, universal”) and Apostolic Church”. The Orthodox Church claims that it is today the continuation and preservation of that same Church.
A number of other Christian churches also make a similar claim: the Roman Catholic Church, the Anglican Communion, the Assyrian Church and the Oriental Orthodox Churches. In the Orthodox view, the Assyrians and Orientals left the Orthodox Church in the years following the Third Ecumenical Council of Ephesus (431) and the Fourth Ecumenical Council of Chalcedon (451), respectively, in their refusal to accept those councils’ Christological definitions. Similarly, the churches in Rome and Constantinople separated in an event known as the East–West Schism, traditionally dated to the year 1054, although it was more a gradual process than a sudden break. The Church of England separated from the Roman Catholic Church, not directly from the Orthodox Church, for the first time in the 1530s (and, after a brief reunion in 1555, again finally in 1558). Thus, though it was united to Orthodoxy when established through the work of Saint Augustine of Canterbury in the early 7th century, its separation from Orthodoxy came about indirectly through the See of Rome.
To all these churches, the claim to catholicity (universality, oneness with the ancient church) is important for multiple doctrinal reasons that have more bearing internally in each church than in their relation to the others, now separated in faith. The meaning of holding to a faith that is true is the primary reason why anyone’s statement of which church split off from which other has any significance at all; the issues go as deep as the schisms. The depth of this meaning in the Orthodox Church is registered first in its use of the word “Orthodox” itself, a union of Greek orthos (“straight” “correct” “true” “right”) and doxa (“glory” as in Doxa Patri, “Glory to the Father”).[