I'm going to be honest here: I know precious little about the Orthodox Church. Because of this, it will be a long time before you see a Bible Brain article directly addressing it. Nevertheless, when I have addressed Catholicism on God Squad Apologetics, several Orthodox people did often attack those posts, making the same claims the Catholics do. This is actually quite frustrating, as I usually can't tell who's who.
One of the similarities Catholics and Orthodox have is that they both oppose what they call "Protestantism". This is basically a loaded term for Christianity. I said loaded because the term "Protestant" assumes Protestants have something to protest, effectively claiming "we came first, you went rogue. We're right, you're non-players".
Of course, in reality, the main point of the Reformation was to return the Church to the word of God. Since the Bible is widely available today, with several English translations being both accepted, and even produced by the Catholic Church, it is not hard to see that it is actually the Catholic Church that is unbiblical. Although I would be labelled a "Protestant" (and although many Christians are comfortable accepting that label), I thoroughly reject it. I see it as sheer arrogance to assume my faith exists only to protest another. My beliefs stem from the Bible, and I defend them with the Bible. Beliefs I hold that I cannot defend with the Bible to at least a significant degree, I remain silent on. For example, you won't find me debating the rapture any time soon, because despite studying it, I can't wrap my head round it. I don't often mention Cessationism, because although I can defend that it is a valid view, I could not definitively say it is the correct view.
This is where Catholics and Orthodox tend to link arms. I've even seen a few of them call themselves "Cathadox" (though ironically, I have also been scolded for using the same term). Basically, whereas Christians believe in Sola Scriptura (the belief that the Bible is the ultimate, sufficient and only infallible authority in Christianity), "Cathadox" are united in their opposition to it.
There are some areas where they are not so united, however. In a debate on this ministry, one Orthodox believer, whom I'd initially believed to be Catholic, said "The Orthodox Church is the Church of Acts. Catholics were part of that Church before they wanted papal superiority and were kicked out almost 1,000 years ago."
I admit my response was not the wisest. I said this was amusing, since they make the opposite claim, which I do admit the fallacy in, but nevertheless, it is the response I gave. This lead to the quote you see in the header image: "It's not that difficult to prove that Orthodoxy has the proper claims and Rome does not. But that would require you to have read even a single book on the topic of ancient Christian history, which I don't genuinely believe you've done."
For added context, this Orthodox person had also commented repeatedly on the same post. We had the usual back and forth discussion where he tried to avoid the Bible and I tried to show that it is the inspired, infallible word of God, making it the most authoritative work in the world. Just like Catholics, his method was to claim I don't know anything about Church history, as if the Apostles would have preached something different to the Bible, or successive generations had enough authority to add to, or otherwise alter it (a position thoroughly refuted by verses like Galatians 1:8 and Jude 1:3).
Is he correct? Is it true that I have never read a book about ancient Christian history? As it turns out, no. See, the difference between Catholicism and Christianity is that while Catholics keep appealing to early Church sources to defend their views, Christians can appeal to the earliest Church source. That is, the Bible, which I have read. Oh, and did I mention we all agree that this source is literally breathed out by God (2 Timothy 3:16-17; 2 Peter 1:20-21)?
With Catholics, it's obvious why they prefer to defend their views with the fallible word of sinful man over the infallible word of God. It's because the infallible word of God is opposed to their man-made doctrines, whereas fallible men can be twisted so that it looks like they believed these things, or maybe they even did legitimately believe them. With Orthodox, I am forced to assume the same is true. After all, if Orthodox agree with the Bible completely, why would they so frequently ally with Catholics to oppose "Protestants" for defending it?
However, God's commands make me wiser than my enemies, for they are always with me. I have more insight than all my teachers because God's decrees are my meditation. I understand more than the elders because I obey God's precepts. The revelation of God's words brings light and gives understanding to the inexperienced.
In case it wasn't obvious, those words aren't technically my own. They are the slightly altered words of Psalm 119. I say slightly (and I cannot stress this enough, please go and read the chapter for yourself) because the Psalm is speaking to God as a prayer, whereas obviously I could not tell my audience that their word brings light. What I can say is this: The Bible testifies to its own sufficiency. If you know your Bible, you know everything you need to know about Church history. Enough to outshine your enemies, your teachers, and even the ancients.
This is because God, unlike any human being, is infallible. Need I defend this claim? Perhaps an atheist would disagree, but I never expect a Theist to say God could ever make a mistake, or that man can do better than God, and when I do find those Theists, my respect for them drops far lower than I'm proud to admit. Cathodox should acknowledge that God is greater than man, and so you can learn more from His word than you can from man's word.
Thus, it doesn't matter who you find that preaches a specific thing at any point in history. If a teaching disagrees with the Bible, it is heretical, and to profess it is to go against God Himself. If a teaching does not agree with the Bible, but does not necessarily disagree with it, Christians are free to use their own judgement about it, and have no right imposing it on each other. If a teaching agrees with the Bible, it is redundant, because it is in the Bible. That which does not have authority from the Scriptures, we can as easily despise as approve, for among the things that are plainly laid down in Scripture are to be found all matters that concern faith and the manner of life. If you have a Bible, and have the Holy Spirit residing in your heart, you need no one to teach you. Oh yeah, and by the way, I directly quoted Augustine and Jerome in the above paragraph, both of whom preceded Martin Luther by centuries.
The above goes for me, too, by the way. You don't need me to teach you anything. Anything I say about the Bible may be true, at which point you don't need me because you have the Bible, and you have the Holy Spirit. It may be neutral, at which point to you be yours and to me be mine. You don't need to believe me. Or what I say may be false, at which point I don't just demand you reject it, but beg that you correct me. I'm not an authority figure. I'm an apologist and an internet preacher. My job is to learn the truth, then try to convince others to accept it. If I ever set myself above the scriptures, or even claim equality with it, I have failed, I need to repent, and you need to reject me.
And so I encourage you not to take me at my word, but to take God at His. It will not be me you face on Judgement Day. Neither will it be the Pope. Neither will it be whoever leads the Orthodox Church (and I didn't even bother to look that up, because I wish to reinforce the point that my knowledge of that religion could not fit on a leaflet sufficient to pass a high school class). The one you face on Judgement Day will be Yahweh, the Lord God of Israel, and so you better know His word, and know it well. Surrendering your God-given right, nay, responsibility to test all things and hold on to that which is good (1 Thessalonians 5:21) is a guaranteed way to absorb any false teaching, and this will not go down well on Judgement Day. Trust God, not man, and especially not men who tell you to trust them over God.