Number of Worldwide Members:
There are an estimated 225-300 million Christians in the Eastern Orthodox Church worldwide. The umbrella of Eastern Orthodoxy includes the following churches: British Orthodox; Serbian Orthodox; Orthodox Church of Finland; Russian Orthodox; Syrian Orthodox; Ukrainian Orthodox; Bulgarian Orthodox; Romanian Orthodox; Antiochian Orthodox; Greek Orthodox; the Church of Alexandria; the Church of Jerusalem; and the Orthodox Church in America.
Eastern Orthodox Church Founding:
Until 1054 AD Eastern Orthodoxy and Roman Catholicism were branches of the same body—the One, Holy, Catholic and Apostolic Church. Divisions between these two branches of Christendom had long existed and were constantly increasing. The widening schism was caused by a mix of cultural, political, and religious differences. In 1054 AD a formal split occurred when Pope Leo IX (head of the Roman branch) excommunicated the Patriarch of Constantinople, Michael Cerularius (leader of the Eastern branch), who in turn condemned the pope in mutual excommunication.
The churches remain divided and separate to the present date.
Prominent Eastern Orthodox Church Founders:
Michael Cerularius was the patriarch of Constantinople from 1043 -1058 AD, during Eastern Orthodoxy's formal separation from the Roman Catholic Church. He played a prominent role in the circumstances surrounding the Great East-West Schism.
For more about Eastern Orthodox History visit Eastern Orthodox Church - Brief History.
Geography:
The majority of Eastern Orthodox Christians reside in Eastern Europe, Russia, the Middle East, and the Balkans.
Eastern Orthodox Church Governing Body:
The Eastern Orthodox denomination consists of a fellowship of self-governing churches (governed by their own head bishops), with the Ecumenical Patriarch of Constantinople holding the honorary title of first in order. The Patriarch does not exercise the same authority as the Catholic Pope. The Orthodox churches claim to exist as a theologically unified communion of churches with the Scriptures, as interpreted by the seven ecumenical councils, as their sole authority and Christ as the head of the church.
Sacred or Distinguishing Text:
The Holy Scriptures (including the Apocrypha) as interpreted by the first seven ecumenical councils of the church. Eastern Orthodoxy also places special importance on the works of early Greek fathers such as Basil the Great, Gregory of Nyssa, and John Chrysostom.
Notable Eastern Orthodox Christians:
Patriarch Bartholomew I of Constantinople (Dimitrios Arhondonis), Cyril Lucaris, Leonty Filippovich Magnitsky, George Stephanopoulos, Michael Dukakis, Tom Hanks.
Eastern Orthodox Church Beliefs and Practices:
The word orthodox means "right believing" and was traditionally used to signify the true religion that faithfully followed the beliefs and practices defined by the first seven ecumenical councils (dating back to the first 10 centuries). Orthodox Christianity claims to have fully preserved the traditions and doctrines of the original Christian church established by the apostles.
For more about what Eastern Orthodox Christians believe visit Eastern Orthodox Church - Beliefs and Practices.
(Sources: ReligiousTolerance.org, ReligionFacts.com, Orthodox Christian Information Center, and Way of Life.org.)