EDIT: It should be obvious, but of course the title refers to the Christian Orthodox Church, as opposed to Catholicism and Protestantism.
I'm exploring the Orthodox Church and the big "selling" point seems to be, "It's the most original form of Christianity, as set up by the Apostles!"
How accurate is that statement?
I know we don't have much about the Church from the 1st century (other than throwaway lines in Paul's Epistles), and I realize that a lot of doctrine was defined by the Ecumenical Councils beginning in 325. But, other than that...?
The Orthodox Church has most certainly not maintained the same doctrines and practices over the past 2000 years. The two clearest examples of this are its adherence to Monophysitism in the 6th c. and the Iconoclastic Controversy of the 8th and 9th c. In both cases, official church practice deviated both from that which it had practiced prior and from that which it would practice after.
"It's the most original form of Christianity, as set up by the Apostles!"
I have no doubt the Orthodox Church itself believes this to be true, but there are few current Christian sects which would not make this claim. Nor is there a historical answer. It's not a question of facts, it's a question of the interpretation of those facts, and comparative weighting. That is, are all deviations from "original" doctrine of the same value, or, for example, do they count more if they concern the Trinity or the nature of Christ? The answers depend on belief, not raw data.
Unlike Roman Catholicism as we know it today, Eastern Church was always very diverse, with many different points of view co-existing together, and with many "centers of faith" (including several different "Popes") with their different distinct traditions, both practical, liturgical, and theological. Sometimes certain streams would diverge from each other, and even stop being in communion; sometimes they would merge together, and co-exist as a braid of different traditions.
As in regards to the claim of being "the most original form of Christianity": well, in some aspects it is more archaic, and thus, arguably, closer to the initial apostolic church. But this archaic heritage of the early church is often covered by layers and layers of Byzantine aesthetics, and then thick layers of local national traditions. If you are really into liturgic history, you will certainly discover with pleasure that some aspects of early Christian traditions that are all but lost in Western Christianity, are still alive in the East, even if in a very vestigial form, and with the original meaning often lost and forgotten. At the same time, I am not quite sure a casual observer would necessarily be able to notice that.