Dzheremi, with all do respect how do you know that Cyril was not teaching heresy himself? Yes he used the Bible to support his position, but so did the other people he was arguing against. How do you know that the teachings of this man were Truth and the others false? Please do not refer to the opinion of more men or popular vote in your answer.
What is the point of such questioning but to slander people and attempt to induce confusion and division where there is none? "How do you know that...?" is not a sincere question, or at least not a sincerely answerable one, given the context in which it is invoked and the constraints you are attempting to place on the answers so that they may be acceptable to you as a Mormon. Again, the problem is that such questioning can come from anyone, towards anyone, for any reason. So we can only go by what actually happened in the real world in which we are all living, not your LDS alternate history fantasies where the orthodox Christians are the ones teaching heresies. Enough of that. You may add 'with all due respect', but that point of view deserves no respect. Come on. We cannot have a serious conversation where one side is saying "Tell me why you're saying that, and don't use any of these sources that I don't approve of". When I ask a question about Mormon theology, I
expect you to use Mormon sources because you are a Mormon, not to only use Orthodox Christian sources, or Catholic Christian sources, or any of these other things that you are not. Don't even pretend to be asking me a question if you are going to follow it up with this trash about "the opinion of mere men or popular vote" -- again, as though Joseph Smith is not a mere man. You don't get to decide the terms by which anyone answers you, and I will thankfully reserve all judgment to the holy fathers who decided so much before us (including, but not limited to, the Bible you are using by which you dare to give heretics wiggle room so as to thoughtlessly slander those same fathers).
If I recall correctly, it is a charge against the Muslims by the great Syriac Orthodox saint Bar Hebraeus that they demand from us an apology for our religion not from our scriptures, but from
those which they recognize. From your example, the same can be said of Mormons. So while I can say that HH St. Cyril taught rightly (you need only look into the Council of Ephesus and the anathemas against Nestorius accepted there, or any of HH's classic works in theology such as
That Christ Is One or
On The Incarnation), and back it up by looking at what actually happened (e.g., the condemnation of Nestorius and his party), what's the point of any of this if it is only going to be answered with another pseudo-question along the lines of "But how can you know that the exact opposite isn't what's true?"
Because that's not what actually happened, and I live in the real world. That's how. Feel free to believe in whatever you want to instead, but there is only one set of things in the list of 'what if' possibilities that
actually happened (Nestorius was condemned). I recognize that to you this is just more evidence of the extremely early 'apostasy' of Christianity, but I just want you to recognize (and others who might read this to recognize) that the alternate history you are presenting is one in which the Council of Ephesus got it wrong, Nestorius is correct, and Mormons are apparently followers or advocates of Nestorius. This man who infamously said, in objecting to calling St. Mary Theotokos, that God could never have been a baby of two months old. That was his level of theological understanding when presented with quite standard festal proclamations of the Epiphany that "Christ was born days ago, and today He is baptized by John" (to quote St. Gregory of Nyssa). The trouble for him, and apparently for Mormons who wish to rehabilitate him for some reason, is that we
do say that Christ was baptized in the Jordan. That's scriptural. That happened. And Christ was born, and hence was a baby for a time. There is no shame in that but that some people have wrong theology which makes them uncomfortable with the child Christ. But we are not uncomfortable, and Nestorius was wrong then, is wrong now, and will be wrong forever. Not because St. Cyril says so alone (since in fact he does not say so alone; he is far from the only saint to object to Nestorius), but because that is the consistent witness of 2,000 years of Christianity that these things happened, not other things that are more in keeping with what wrong-headed men like Nestorius or Joseph Smith can understand of God.
Hence it is traditional to understand instead that the honor paid to St. Mary which gave Nestorius such trouble is therefore theologically rooted in our recognition of the divinity of Christ Himself (and the one who cannot proclaim the same is therefore suspect, theologically speaking). As we sing in the Sunday psali in the Coptic Orthodox tradition "He who abides in light Whom no one can approach showed us His miracles / and you fed Him."
Anyone who has a problem with that has a problem with Christianity and traditional Christian theology. Mormonism, apparently like Nestorius, has a problem with that, and you do not deal with it by trying to turn the tables on those who are teaching rightly in order to exonerate the ancient heretics for the sake of your own more modern heretics.
. If not, I wish you well regardless 
