Michael,
Here's my best effort:
It's as good to speculate how we would "fit" into the ECFworld, as is it to speculate how they would fit into the 21st century church. Why shouldn't it make as much sense as anything to think that were Athanasius alive today--and had all the benefits of a modern theological education, access to church history, and a readable Bible--assuming that he was the same kind of person (first a lover of Christ and the Bible) that he would stand with us?
It is so terribly anachronistic to bring someone from another time into this one, or project ourselves backward into that one. Because, we cannot fathom just how much we are products of the time and circumstances in which God settled us. Providentially, THIS is the optimal time (so far as I know) for me to live; for in any other time, perhaps I would have ended up an unbeliever. Change one thing about my childhood, and maybe I'm an atheistic or theistic evolutionist IN MY OWN TIME. But that's not what happened.
All we can really say is that we understand that certain theological errors are not FATAL to one's faith. And some of that has to do with where God has placed us geographically and temporally. There's the big-picture, and the small-picture. Some people today are very "childish" or "childlike" in their faith. They only know that they get fed a diet every week in church, and they don't deeply interpret that faith. Are they being fed good diet or bad? Will it sustain them, or won't it? They may not know that 'til Judgment Day.
Here's the big picture: the ECF's were living in the "infancy" of the NT church. They were "children" in their Providential location. God put them there, and that's all there is to it. That observation in no way belittles them, or shortens their INTELLECT. These were giants of faith and learning, when we compare men-to-men. But there's this other layer, this "temporal" layer.
One great Providential "problem" the early church had to face was that in about two generations, they lost a portion of their "Old Testament" mind. They lost the ability in Hebrew. Jerome almost singlehandedly resurrected (for a time) Hebrew scholarship. But worse than the language problem was the devaluing of the OT. The ECF stumbled in how to relate to OT revelation--not entirely, because they were trying to "find Christ" there, but "allegorical" exegesis displaced true "typology." Now, if you have for all intents/purposes a truncated canon, your overall biblical theology is going to suffer also. So, the ECFs are laboring under a variety of social effects that impact how they get into the Bible.
The Jews had joined the Romans in persecuting the Church in the first century. This drove the initial wedge. Then, the Jews were persecuted wildly by the pagan Romans, and their country wiped out. Hebrew writing, and probably even study of the OT even in Gk., is bound to be minimized culturally in the church. Then, the church is wildly persecuted by Rome for two more centuries, and the writings that they care most to save are the NT writings. These are social effects that work their way out in priorities for the church to study the Bible.
Then, there is the order of the heretical attacks on the church. The church has to prioritize its in-depth study and defense of the truth. The best minds leave other important studies for the most pressing studies. So what happens in those other areas? Variety, and inevitably mistake and error in the nature of the case. But this or that problem is not as DEADLY as the problem of the deity and humanity of Christ. Get those wrong, and the other problems will not have a chance to kill the church off--it'll already be dead.
That is a lot to chew on, I think, so I'll just leave you with this illustration:
Back to the "church-as-child" analogy, the big-picture (similar to the big-picture OT Israel as God's "son"). A kid gets fed every day. He doesn't know diddly about how that food gets to the table, he just likes to eat. And the big-picture is that God sees to it that the church gets nourished.
That same kid doesn't have a very well-formed picture of the world either. He's wrong about lots of things, or better put, he's mixed up about many things. He has some facts in his head and some fantasy, and its not easy for him to tell the difference. He's not even thinking about it. But he knows where the table is. And when he's set before him a bowl of gravel, he spits that out, and tries to find his bowl of porridge.
Meanwhile, he grows and he sorts out his thinking on lots of other things. He gets settled on many things, like who's in charge, how do I fit in this scheme, time to change my diaper, car-seat means environment change, etc. But for a long, long time, he got it in his head that he'd like to fly like a bird, as soon as he could. If he's prevented from going out, and climbing up on a roof, chances are he won't try flapping his arms and jumping--until he's learned a little bit about gravity and his limitations. God doesn't Providentially give the church more than she can handle, and he lets it keep some mixed up notions, because its more important to get the bowl of porridge vs. bowl of gravel thing right.
Why didn't God just zap the church with perfect knowledge? "That's how I would do it!" That's the silly question lots of people ask. He didn't (obviously), and trying to sell the idea that all the childish notions of childhood were somehow "instinctually true" and accurate is ridiculous. Rome and EO often sound like they say this sort of thing. Or, Rome (more than EO I think) admits that ideas have morphed over time, but still the church didn't have ANY WRONG notions, at the start, just LESS FORMED but still the same as today. Of course, EO also holds wrong notions, not least for which reason she was strongly influenced by Greek/Gnostic philosophy early on. When generation after generation no reformation happens, no getting back to the Bible for corrections, no church will grow properly. This was the benefit of the Reformation--getting back to reading and understanding the Bible.
But maybe then, the child still thinks for a long time that he's like a magnet, and the earth is like a big refrigerator, that's why he's stuck to it. You can see that if he grows up, thinking he can learn to fly by jumping off buildings, or thinking that he's attracted to the earth like a magnet to a fridge, eventually that's going to create problems for him. He needs to think clearly on these things.
And he does so how? By learning. By a system of learning, that is integrated to the world of reality. He keeps at it, and he corrects. Because the basic instruction set is reliable. Unless he tosses it over, and chooses a flattering instruction set that validates his errors.
Hope this is a helpful bit. I haven't time to review it.