Peter Brown acknowledges that "it is still difficult to discover [Constantine's] intentions" for his conversion as well as "the extent of his ambition on behalf of the Christian faith" (Through the Eye of a Needle, 32). Nevertheless, he gives what seems to me to be a judicious and accurate assessment.
Constantine did not act out of political calculation. He did not perceive that Christianity was on its way to becoming a majority religion in any part of his empire--not even in the more densely Christianized East and certainly not in the Latin West, where Christianity seems to have made considerably less progress (32).
Constantine was prepared to devote great pains to keeping the worship of [the Christian] God immune from error and division....[He]patrolled the Christian episcopate ceaselessly to make sure that the worship they offered to God was correct and pleasing to Him.
But that was almost all Constantine was prepared to do. The non-Christian subjects of his empire were left largely to themselves; the Roman West remained a predominantly pagan world.
Thus Brown says,
Constantine certainly thought of himself as a Christian. But he was not a Christian of the middle ages, nor was he even a Christian of the late fourth century. It was enough for him that the Christian God should be recognized, that Christians should no longer be persecuted, and, above all, that the Christian clergy should be privileged and protected (33).
Constantine was not the angel that Eusebius made him out to be. Nevertheless, it is historically inaccurate to assume that Constantine magically made Christianity into an easy and attractive religion for the Roman Empire. In fact, at the end of this deep study, Brown concludes,
Faced with massive losses of income and by a widespread weakening of its authority in large areas of the West, the imperial structure inherited from the fourth century showed no intention of surrendering. Instead, the empire--the Respublica--went down fighting. And it went down fighting as a frankly secular institution. Government circles made plain that they would not yield any of the prerogatives of the Roman state to the Christian bishops (529).
This is instructive for Christians of all political persuasions today.