Quoted article from: https://christiantheology.wordpress.com/2009/01/28/can-eastern-orthodox-prove-theyre-the-one-true-church/

Eastern Orthodox adherents are very passionate about their faith. While that is all fine and good, there are a number of problems with that faith and their practices, and I have found the answers to these problems evasive, confusing, or non-existent. Since this church also claims to be the “One True Church,” it is disappointing to find its scholarship rather weak and misguided.

While there are a number of errors and practices of Orthodoxy that need to be addressed, I believe that foundational truths, those things on which the whole of this church should stand or fall, should be challenged first.

The first and ultimate question I have for Orthodox believers is, can they prove that Eastern Orthodoxy is the “One True Church” and that all others are outside the faith and apostate? While there a number of arguments that Orthodox believers postulate, none of these arguments, under scrutiny, hold water.

First, they state that they are the one true church because they hold the line of Apostolic succession. That is, since the “True Church” will consist of an unbroken line of Apostles from Peter and Paul until today, they claim that they are the True Church because their bishops are part of that unbroken line.

The first problem with this belief is that more than a dozen churches, Orthodox, Catholic, and Protestant, also claim this line as proof that they are the one true church. In each of these churches they have a public listing of their unbroken line of bishops. Why then is the Orthodox right and them wrong?

The Orthodox claims the rightful line of Apostolic succession because they have not apostatized. And because the other churches have apostatized, they are not the True Church.

But how do we know that those other churches have apostatized and not the Orthodox Church? Because the Eastern Orthodox are the True Church, of course. They are the ones who have been given the Truth, and when others disagree with them, those other churches are wrong. And for the Orthodox, the Bible is not the final authority, the Church is. Therefore, what they say is the truth, is the truth. There is no higher authority or objective standard to which they appeal. Thus, when the church says that they are the True Church, it’s true, because they are the Final Authority, and they are the Final Authority because they are the True Church. This is a rather obvious tautological statement, and completely meaningless.

Can we appeal to the Bible? No. Not at all. As any non-Orthodox believer soon finds out, the Bible can only be rightly interpreted by the Eastern Orthodox believer, because they are in the Truth, and no one else is. Thus, any passage of scripture that we appeal to is rejected as a wrong interpretation. According to Orthodoxy, Scripture is not only interpreted and defined by them, they wrote it.

Can we then appeal to truth or logic? Again, no. For the Orthodox, truth can also only be interpreted by them. “Truth, to the Orthodox,” according to one official Eastern Orthodox web site, “is not a proposition or conclusion; Truth is a Person, a living experience accessible in the communion of the Church and expressed in the Scriptures, the councils, and the theology of the saints. Even the Ecumenical Councils needed to be received as normative by the body of the Church. Ultimately, there are signs that point to truth, but none of these signs is a substitute for a free and personal experience of truth, which is encountered in the sacramental community of the Church.” (emphasis mine). Again, truth is defined within the confines of the Orthodox Church.

However, we have to understand that the Orthodox know this “free and personal experience” of truth is true because “Truth is a Person,” namely the Holy Spirit. While we can agree that the Holy Spirit is the Spirit of truth (Jn. 16:13), the Orthodox believe that He works only in their lives, and gives them the truth by experience. Or as the Mormons would call it, a “burning in the bosom.” So if their experience were to tell the Church that God is dead, and the church all agreed, then God would then be dead, for the Holy Spirit has given them the truth.

To conclude, the Orthodox lay claim to the Holy Spirit, truth, the interpretation of the Scriptures, the final authority, and Apostolic succession, because they are the True Church. And they are the True Church because they lay claim to all these things. None of these are proofs, and many other churches also claim these same proofs as their own.

But the real confusion is how and why do intellectual and discerning Christians, even staunch Calvinists, leave the Protestant faith to follow Orthodoxy? While I am quite sure of my assessment of Eastern Orthodoxy, I honestly wonder if I am missing something. Why would any Christian follow Orthodoxy given their beliefs?

Thus I ask if anyone can give an answer. What makes the Eastern Orthodox Church the True Church? Where is the proof? What makes Orthodox claims true – which are the same baseless claims as many other churches – and those other church claims false?

There is one request I have for anyone who answers. Your answer cannot be that Orthodoxy is true because Protestants are false. Proving one person wrong does not prove you right. We can both be wrong, but we cannot both be right. The question is not whether or not Protestants, Catholics, Anglicans, or Mormons are false, the question is how is Orthodoxy right? And more specifically, prove that Orthodoxy is the True Church when many others make the same claim.