Silly question. Because we have objective proof that it is! :)
Our liturgy predates all other extant liturgies, and it just happens to be the liturgy used at the same time the Church canonized the Bible. Same mindset, same liturgy, same Church. To the best of my knowledge there wasn't any other at that time. It has to be pure because it canonized the NT, whose selection of books you use as scripture.
But, on the other hand, there are those who claim that the Church didn't do it (despite documents to the contrary), but claim, without any proof whatsoever, that the HS put it all together and simply handed it to the bishops, which of course, is nothing short of a fairytale.
These are the same people who persistently avoid to comment about the evidence of the existence of prehistoric animals because such a reality, and fact, interferes with (even threatens) their fairy tale beliefs.
Purity of Eastern Orthodoxy does not require an a priori belief. It is not a solipsistic fantasy based on self, nor on Gnostic self-righteousness. It is a historical fact that it is the most unchanged and oldest form of organized Christianity, and reflect in its totality the worship and the teaching of the Church that claimed the same heritage from the beginning when it canonized the New Testament.
How's that poultry, Dr. E?
You mean the New Testament that is imperfect and imprecise and some of it just plain wrong?
That New Testament?
“objective proof that it is”
Objective proof is inductive reasoning (a posteriori) and that only renders “probable” conclusions, therefore, the correct statement is “I believe....”. “Pure Christianity” is a belief system and since this is a fallen world and we are fallen people saved by grace, we can’t know that our particular faith system is “pure”. Paul says, “we see through a glass darkly...”