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The Eastern Orthodox Church, officially the Orthodox Catholic Church, and also called the Greek Orthodox Church or simply the Orthodox Church, is the second-largest Christian church, with approximately 230 million baptised members. It operates as a communion of autocephalous churches, each governed by its bishops via local synods. The church has no central doctrinal or governmental authority analogous to the head of the Catholic Church (the pope). Nevertheless, the Ecumenical Patriarch of Constantinople is recognised by them as primus inter pares ("first among equals"), a title formerly given to the patriarch of Rome. As one of the oldest surviving religious institutions in the world, the Eastern Orthodox Church has played an especially prominent role in the history and culture of Eastern and Southeastern Europe.

Eastern Orthodox theology is based on the Scriptures and holy tradition, which incorporates the dogmatic decrees of the seven ecumenical councils, and the teaching of the Church Fathers. 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 practises the original Christian faith, as passed down by holy tradition. Its patriarchates, descending from the pentarchy, and other autocephalous and autonomous churches, reflect a variety of hierarchical organisation. 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. The Virgin Mary is venerated in the Eastern Orthodox Church as the God-bearer and honoured in devotions.

The Churches of Constantinople, Alexandria, Jerusalem, and Antioch—except for some breaks of communion such as the Photian schism or the Acacian schism — shared communion with the Church of Rome until the East–West Schism in 1054. The 1054 schism was the culmination of mounting theological, political, and cultural disputes, particularly over the authority of the pope, between those churches. Before the Council of Ephesus in AD 431, the Church of the East also shared in this communion, as did the various Oriental Orthodox Churches before the Council of Chalcedon in AD 451, all separating primarily over differences in Christology.

The Eastern Orthodox Church is the primary religious denomination in Russia, Ukraine, Romania, Greece, Belarus, Serbia, Bulgaria, Georgia, Moldova, North Macedonia, Cyprus and Montenegro. Roughly half of Eastern Orthodox Christians live in the post Eastern Bloc countries, mostly in Russia. The communities in the former Byzantine regions of North Africa, the Eastern Mediterranean are among the oldest Orthodox communities from the Middle East, which are decreasing due to forced migration driven by increased religious persecution. Eastern Orthodox communities outside Western Asia, Asia Minor, Caucasia and Eastern Europe, including those in North America, Western Europe, and Australia, have been formed through diaspora, conversions, and missionary activity.