Eastern Orthodox theology is based in Nicene Creed, and the Eastern Orthodox Church teaches that it is the One, Holy, Catholic and Apostolic church established by Jesus Christ in his Great Commission, and that its bishops are the successors of Christ's apostles. It maintains that it practices the original Christian faith, passed down by sacred tradition. Of its several patriarchates four reminiscent the pentarchy, while its autocephalous and autonomous churches reflect a variety of hierarchical organisation. Of its innumerable sacred mysteries, it recognises seven "major sacraments" of which the Eucharist is the principal one, celebrated liturgically in synaxis. The church teaches that through consecration invoked by a priest the sacrificial bread and wine become the body and blood of Christ. Virgin Mary is venerated in the Eastern Orthodox Church as the Mother of God, honoured in devotions.
The contemporary Eastern Orthodox Church shared communion with the contemporary Roman Catholic Church until the East–West Schism in AD 1054, triggered by disputes over doctrine, especially the authority of the Pope. Prior to the Council of Chalcedon in AD 451, also Oriental Orthodox churches shared in this communion, separating primarily over differences in Christology.
The majority of Eastern Orthodox Christians live in Eastern Europe, Greece, and the Caucasus, with smaller communities in the former Byzantine regions of the eastern Mediterranean, Africa, and to a decreasing degree also in the Middle East due to persecution. There are also many in other parts of the world, formed through diaspora, conversions, and missionary activity.
In keeping with the Church's teaching on universality and with the Nicene Creed, Orthodox authorities such as Saint Raphael of Brooklyn have insisted that the full name of the church has always included the term "Catholic", as in "Holy Orthodox Catholic Apostolic Church". The official name of the Eastern Orthodox Church is the Orthodox Catholic Church. It is the name by which the church refers to itself in its liturgical or canonical texts, in official publications, and in official contexts or administrative documents. Orthodox teachers refer to the Church as Catholic. This name and longer variants containing "Catholic" are also recognised and referenced in other books and publications by secular or non-Orthodox writers.
The common name of the Church, Eastern Orthodox Church, is a shortened practicality that helps to avoid confusions in casual use. From ancient times through the first millennium, Greek was the most prevalent shared language in the demographic regions where the Byzantine Empire flourished, and Greek, being the language in which the New Testament was written, was the primary liturgical language of the church. For this reason, the eastern churches were sometimes identified as "Greek" (in contrast to the "Roman" or "Latin" church, which used a Latin translation of the Bible), even before the great schism. After 1054, "Greek Orthodox" or "Greek Catholic" marked a church as being in communion with Constantinople, much as "Roman Catholic" did for communion with Rome. This identification with Greek, however, became increasingly confusing with time. Missionaries brought Orthodoxy to many regions without ethnic Greeks, where the Greek language was not spoken. In addition, struggles between Rome and Constantinople to control parts of southeastern Europe resulted in the conversion of some churches to Rome, which then also used "Greek Catholic" to indicate their continued use of the Byzantine rites. Today, many of those same Roman churches remain, while a very large number of Orthodox are not of Greek national origin, and do not use Greek as the language of worship. "Eastern", then, indicates the geographical element in the Church's origin and development, while "Orthodox" indicates the faith, as well as communion with the Ecumenical Patriarchate of Constantinople. There are additional Christian churches in the east that are in communion with neither Rome nor Constantinople, who tend to be distinguished by the category named "Oriental Orthodox". While the Church continues officially to call itself "Catholic", for reasons of universality, the common title of "Eastern Orthodox Church" avoids casual confusion with the Roman Catholic Church.
The first known use of the phrase "the catholic church" (he katholike ekklesia) occurred in a letter written about 110 AD from one Greek church to another (Saint Ignatius of Antioch to the Smyrnaeans) The letter states: "Wheresoever the bishop shall appear, there let the people be, even as where Jesus may be, there is the universal Church." Thus, 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 Eastern 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. In the Eastern 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 Eastern 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.