Additionally, the Protestant Reformers did not view the Anabaptists as part of the Reformation, although historically their origin is tied in with Luther's break from the Papacy.
The split between the Roman and Eastern churches occurred officially in 1054 (pre-Reformation). The primacy of the bishop (Pope) of Rome vs. the patriarches of the East was one major issue. The two branches also differ on celibacy for priests, when to celebrate Christ-mass (December 25 in the West and January 6 in the East) as well as certain other issues. The info below may shed some additional light.
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The Eastern Orthodox Churches
The assertion of the early Christian church that Jesus was the incarnation of God on earth created a theological dilemma. If Jesus was God, could he also be a human being, a man whom other humans could emulate because he had endured human suffering? One solution was to say that Jesus was both true God and true man--that, in fact, God can exist in the persons of the Father, the Son, and the Holy Spirit and still be One. This concept was debated at church councils in Nicaea in 325, Constantinople in 381, and Ephesus in 431, and was reasserted at the fourth ecumenical council of Chalcedon in 451. It eventually became known as the doctrine of the Holy Trinity.
But not all Christians agreed, fearing that it implied a division between the human and divine in Jesus. The split deepened over the next two ecumenical councils, and these "non-Chalcedonian" Christians separated into other churches known as Monophysite for their belief that Christ has only one nature, although they do accept his humanity. The Monophysite churches included the
Coptic Orthodox Church of Egypt
Syrian Orthodox Church, sometimes called Jacobite, for the 6th century Bishop Jacob Baradaeus.
Ethiopian Orthodox Church, linked until 1959 with the Copts.
Syrian Orthodox Church of the Malabar (South India), also known as the St. Thomas Christians, or Mar Thoma, for the Apostle Thomas, who they claimed as their founder and who supposedly suffered martyrdom near Madras.
In addition, between the 8th and 11th centuries, a schism developed between the Roman and Eastern Orthodox branches of the Church, which came to a head, known as the Great Schism, in 1054. The main point of contention was the Roman Church's insistence that the Holy Spirit, the Third Person of the Trinity, proceeds from the Father and the Son, rather than just from the Father.
The Eastern Orthodox branch included the Orthodox Churches of Greece, Serbia, and Russia, with modern branches including the Romanian, Bulgarian, Albanian, Ukrainian, and Carpatho-Russian Orthodox. The Orthodox today profess most of the same beliefs as the Roman Catholic Church, but they reject the leadership and infallibility of the Pope, preferring to follow their own local bishops, called metropolitans. (Senior bishops are patriarchs; monasteries are headed by archimandrites.) Eastern Orthodox priests can marry and have families, but bishops were required to be celibate from about the 7th century, and today they must be monks as well.
Orthodox belief emphasizes the Incarnation of God in the person of Jesus Christ. Since God is viewed as the Cosmic Creator, His taking on human form is a great inconceivable mystery. And so the Orthodox use of icons -- representations of the Incarnated God, along with Mary and the saints -- is their way of celebrating that mystery and not mere "idol worship" as Western Christians believed. The iconoclastic movement begun by Pope Leo III in 725 was intended to counter the Eastern belief in the legitimacy of icons (and perhaps as a response to the rapid growth of Islam, which forbade all use of images). However, Leo's precepts were eventually rejected by the Church, and now both Roman and Eastern Christians are free to venerate statues, icons, and other images of Jesus, Mary, and the saints.
Eastern Orthodoxy also emphasizes monasticism and the pursuit of mystical union with God much more than either Roman Catholicism or Protestant Christianity. But with about 225 million practitioners worldwide, the Orthodox make up just a little more than ten percent of the world Christian population.
Source:
http://www.myss.com/worldreligions/Christianity8.asp