OK, we are using common terms, but with seemingly different meanings. Please define "the Church". Also God allows sin and disobedience (which is anything against His will) to happen all the time, but He does not approve of it, Constantine is no exception.
"Let no one do any of the things which concern the Church without the bishop...Wherever the bishop appears, there let the people be, just as wherever Jesus Christ is, there is the Catholic Church." - Ignatius of Antioch, 2nd century
"The episcopate is a single whole, in which each bishop enjoys full possession. So is the Church a single whole, though it spreads far and wide into a multitude of churches as its fertility increases." - Cyprian of Carthage, d.258
In the Orthodox Church, we have an expression. We know where the Church is, but not where it isn't. We believe that the Orthodox Church is the direct descendent of the Church of the Apostles, indeed is the exact same Church. The Church is not a mere organization, but a living entity infused with the life of God. This is not due to any merit of the members of the Church, but due to the will of God. But, this does not necessarily mean that everyone within the visible Church will be saved. Thus the need for each of us to continually work out our own salvation. And it doesn't necessarily mean that those outside the visible Church will not also be saved by God.
But in my perspective, the burden seems to be on those who deny that the existence of Apostolic succession and deny the ability of the visible Church to endure through the ages. Where is the evidence that somehow Constantine created something that was not the Church, and that took its place? I just don't see it, and have yet to hear any claims to support this great overturning of God's stated purpose in establishing the Church.
All I've heard, I think, are two propositions:
1 - That because Constantine was irredeemably sinful, and leader of the state, the Church during his time became irredeemably corrupt. And even if I grant the first premise, which I have no reason to, I just don't see how this leads to the second.
2 - That by being legitimized by the state, and accepted as the religion of the state, the Church necessarily became corrupted and broke from the teachings and traditions of the Apostles. Again, I don't see the evidence to support this claim. Are you saying that the Church cannot exist except in a state of persecution? How does this square with Christ's command to make disciples of all nations?