You are assuming I believe Constantine was a light switch, the pre-Constantine church one thing, the post-Constantine church suddenly something altogether other. I do not see it like that. I see the pre-Constantine church morphing into the Constantine one, the Papacy years later putting the finishing touches to the morphing process.
You assume the pre-Constantine ECF bishops are equivalent to the original church in the book of Acts. Acts 20:29, 30 would say otherwise. It proves it doesnt take several hundred years for people to backslide from original truths: after Pauls departing Ephesus, grievous wolves would enter in among them, of their own selves men would arise speaking perverse things to draw away disciples after them.
I see the Papacy representing the worst form of the grievous wolves speaking perverse things trend Paul warned about.
Main line Protestantism is more representative of the ECF. True Christians are more Berean than main line Protestants, going further in measuring Christian beliefs against the NT.
The Papacy? It is so far out in the Constantine-Roman-pagan weeds, it is not even funny, not even relevant.
Then when did the great apostasy actually occur? At what date did the beliefs and teachings of the early church become unreliable?