The Eastern Orthodox Church, officially the Orthodox Catholic Church, operates as a communion of autocephalous churches, each governed by its bishops in local synods, although roughly half of Eastern Orthodox Christians live in Russia. The church has no central doctrinal or governmental authority analogous to the Bishop of Rome, but the Ecumenical Patriarch of Constantinople is recognized by all as primus inter pares ("first among equals") of the bishops. As one of the oldest surviving religious institutions in the world, the Eastern Orthodox Church has played a prominent role in the history and culture of Eastern and Southeastern Europe, the Caucasus, and the Near East.
Eastern Orthodox theology is based on the Nicene Creed. The 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, as passed down by holy tradition. Its patriarchates, reminiscent of the pentarchy, and autocephalous and autonomous churches reflect a variety of hierarchical organisation. Of its innumerable sacred mysteries, it recognizes seven major sacraments, of which the Eucharist is the principal one. The church teaches that through consecration invoked by a priest, the sacrificial bread and wine become the body and blood of Christ. The Virgin Mary is venerated in the Eastern Orthodox Church as the God-bearer, honoured in devotions.
The Eastern Orthodox Church shared communion with the Roman Catholic Church until the East–West Schism in 1054, triggered by disputes over doctrine, especially the authority of the Pope. Before the Council of Chalcedon in AD 451, the Oriental Orthodox churches also shared in this communion, separating primarily over differences in Christology.
What unites Orthodox Christians is the catholic faith as carried through holy tradition. That faith is expressed most fundamentally in scripture and worship, and the latter most essentially through baptism and in the Divine Liturgy. Orthodox Christians proclaim the faith lives and breathes by God's energies in communion with the church. Inter-communion is the litmus test by which all can see that two churches share the same faith; lack of inter-communion (excommunication, literally "out of communion") is the sign of different faiths, even though some central theological points may be shared. The sharing of beliefs can be highly significant, but it is not the full measure of the faith according to the Orthodox.
The lines of even this test can blur, however, when differences that arise are not due to doctrine, but to recognition of jurisdiction. As the Eastern Orthodox Church has spread into the west and over the world, the church as a whole has yet to sort out all the inter-jurisdictional issues that have arisen in the expansion, leaving some areas of doubt about what is proper church governance. All members of the Eastern Orthodox Church profess the same faith, regardless of race or nationality, jurisdiction or local custom, or century of birth. Holy tradition encompasses the understandings and means by which that unity of faith is transmitted across boundaries of time, geography, and culture. It is a continuity that exists only inasmuch as it lives within Christians themselves. It is not static, nor an observation of rules, but rather a sharing of observations that spring both from within and also in keeping with others, even others who lived lives long past. The church proclaims the Holy Spirit maintains the unity and consistency of holy tradition to preserve the integrity of the faith within the church, as given in the scriptural promises.
The majority of Eastern Orthodox Christians live mainly in Southeast and Eastern Europe, Cyprus, Georgia and other communities in the Caucasus region, and communities in Siberia reaching the Russian Far East. There are also smaller communities in the former Byzantine regions of the Eastern Mediterranean, Africa, and in the Middle East where it is decreasing due to forced migration because of increased religious persecution in recent years.