And what is the Eastern Orthodox Church which was founded at least contemporaneously, if not earlier, than the Roman Church ?
The Eastern Orthodox, Oriental and Assyrian Churchs are one with the Catholic Church as part of the ONE Apostolic Church.
To answer your question from an Eastern Perspective, the Eastern Orthodox Church is The Eastern Orthodox Church, it is the same as it ever was and it has existed since Pentecost...Rome used to be a part of it.
This is a simplifying statement: From Pentecost until the 11th century there was one Christian Church - it was founded by Jesus Christ. It had grown during that time to include many Bishops, but 5 of them were considered pre-eminent: Rome, Constantinople, Jerusalem, Alexandria, and Antioch. (By way of further detail, the Egyptian Coptics and the Ethopian Orthodox churches broke communion after the 3rd ecumenical council. These churches still exist but are what is known as “Non-Chalcedonian,” meaning they did not accept the rulings of the Council of Chalcedon).
Anyway, of those 5, a seat of honorary primacy was held by the Bishop of Rome, owing largely to the fact that the first Bishop of Rome was St. Peter - who was also recognized as the leader and primary spokesman for the 12 Disciples.
Differences in language and culture (as the Empire expanded) and later philosophy and theology began to isolate Rome from the rest of the Bishoprics, or “Sees.” Friction intensified over several centuries between Rome and the rest of the Sees, which were separated from Rome by the Mediterranean as well as primary language (Latin vs. Greek). This escalated to a particular event in the year 1054.
The Eastern Church has always viewed Church decisions to be made in a conciliar fashion, while the Roman Church evolved as a Monarchy. The Eastern Church as a rule eschews instituting ecumenical decisions made by one man.
For the third time in this posting, and for the benefit of my FReeper brothers and sisters who may be adherents of the Church of Rome, the things I say here are the Eastern Perspective.
Protestantism developed out of the Western Church - out of the Roman Church. There has not been a similar “Reformation” from the Eastern Church.
A little joke that puts that in perspective...
How many Orthodox Christians does it take to change a light bulb?
Change? What is this, “CHANGE?”
I refer to them as Saints in the conventional definition since they were both murdered while spreading the Gospel.