I read an answer here which explained the difference between the various branches of the Eastern Church.
I was thinking as an extension, what is the difference between Eastern Christianity and Protestant Christianity, as both seem to reject the claims of supreme authority from the Catholic Church? What are the major differences between Eastern and Western Orthodox Christianities? Why aren't the Oriental and Eastern Orthodox Churches in communion with each other?
To limit the massive territory of how Protestants differ from Orthodox, I'm going to filter it down a couple times.
First, I'm not going to include points that the Orthodoxy and Catholicism agree on that Protestants (typically) disagree with: that the Eucharist is the body, soul, and divinity of Christ; that Mary is the Mother of God; Apostolic Succession; iconography; sacraments; communion of the saints; rejection of the Protestant solas; contemplative prayer & mystical theology; etc. The reason for this is that you can learn about all these points just from reviewing the Catholic-Protestant debate.
Second, I'm not going to include points that Protestantism and Catholicism agree on that Orthodox disagree with or don't share: original sin, the entire Augustinian framework, the strong emphasis on scholastic theology, a general emphasis on the sacrifice of Jesus' crucifixion rather than the Incarnation as the locus of salvation, some degree of reticience around hesychasm & Palamian theology, filioque, etc.
I could be wrong, but once you cross out all that stuff, I think all you're left with is the Pope, in Catholic understanding the principle of unity of the Church. Of course, Orthodox and Protestants "agree" on this in completely different ways, since Protestants reject the Orthodox understanding of apostolic succession that the Catholic papal ecclesiology requires, so I would be hard pressed to call this a point of true agreement.
The "primary" difference, in my own observation that is,would be that Orthodox believe that the ritual is sufficient and salvific while some Protestants would hold that the ritual is not complete without experience. There are many shades in between of those who have abandoned completely outward ritual and those who completely invest spiritual encounter within the bounds of ritual alone. I've not gone into all the differences of doctrine and position but have given a personal observation over many years.
Catholicism came first, Orthodoxy came second, then Protestantism came third. Because Protestantism formed due to breaking away from Catholicism, you won't find their reasons for breaking away based in Orthodoxy. But given your query about the matter of supreme authority, that in itself shows why Protestantism is distinct from both. The history of authority prior to Protestantism needs to be grasped, and then it should be clear where radical differences lie in that area.
"The pope is the religious leader of Catholics worldwide whereas the Eastern Orthodox Churches comprise several groups, self governing, including the ancient Patriarchates of Antioch, Alexandria, Jerusalem and Constantinople, and the Churches of Greece, Cyprus and Sinai. Patriarchs are their leaders.
From the fourth century the Church of Constantinople – as the Church of the new capital of the Roman (later Byzantine) Empire – ranked second in honour to the Church of Rome, and its leader, or Patriarch, came to be given the title Ecumenical but most of the various groups within Eastern Orthodoxy have Patriarchs.
The Orthodox Church is based upon decrees and dogmas of various ancient Councils before the division of Eastern and Western Christians. There are seven ancient Councils that both Catholics and Orthodox are agreed upon, but after 787 AD, a split arose. The Eastern Church found unacceptable later dogmatic definitions made by the Roman Catholic Church about the Immaculate Conception and Bodily Assumption of the Blessed Virgin Mary, and the infallibility of the Pope. There was also later disagreement about the Western Church statements on the meaning of Filioque (with regard to the Holy Spirit).
Hierarchy of the Orthodox community: Parishes are grouped in dioceses, headed by a bishop. In Russia and Romania dioceses are grouped into larger areas, presided over by a metropolitan. Church groups do have autonomy but this usually coincides with national frontiers, except among Orthodox members who have emigrated, for whom Church organizations normally follow ethnic lines. The larger national Churches are headed by patriarchs, the smaller by archbishops. The Patriarch of Constantinople is recognized as the focus of unity of the Orthodox Churches. The highest authority in the Orthodox Church, the ecumenical council, has not met sine the Council of Nicaea in 787 A.D. A Pan-Orthodox Synod may deal with practical issues in contemporary Orthodox Church life. The Encyclopedia of World Faiths, pp 81-86, Bishop & Darton (Macdonald Orbis, 1987)
The Orthodox respect the pope but as they do not consider his doctrinal statements to be infallible, they will not go along with anything any pope says that does not already conform to the first, early seven Councils. Protestants likewise do not consider papal statements to be infallible, but they don't go by various Councils to determine matters. They use the Bible.
Protestantism agrees with various doctrines established at various Councils but that's only because they are shown to be biblical. It is the inspired word of God - the Bible - that Protestants take as their rule of faith and authority. If it cannot be proven from the holy Scriptures, then they will not go by what any pope or patriarch says (or, they shouldn't!) That seems to be the core difference to me, a Protestant, between both Catholicism and Orthodoxy.
Neither Orthodox nor Protestant Churches are all the same. The most significant difference is the origin: Orthodox Churches have the same age as the Roman church, whereas Protestant churches have split off the Roman church. Most Protestant churches do not have any dogmas except the Bible. Protestants do not pray in front of pictures and to intermediates like saints or Mary; only some pray to Jesus en lieu to God.
