An earlier post mentioned Constantine as "establishing" the Christian Church. I misinterpreted that to mean creating, as opposed to what the poster meant which was making it the established -- i.e. official -- religion. In comnenting, I mentioned the disagreements over the story of the conversion of Constantine. It was a side issue, however, and I was writing on-line. I couldn't give it justice. Here is more thoroughness.
The official version is this:
Before a battle with Maxentius for the control of the empire in the west, Constantine saw a vision of the cross in the sky and heard a voice say: "In hoc signo vincennes". This doesn't mean "a sign pawned in Indiana." It means "By this sign you shall conquer." So Constantine had his troops paint a cross on their shields. He did, indeed, win that battle.
(Various Roman histories give slightly different reports. Some have the vision during a dream; some have a different sign of Christianity. However the details vary, the general idea remains.)
While nobody seems to question Constantine's semi-adoption of Christianity in this battle, there are widespread doubts about Constantine's motivation. After all, the man wasn't baptized until the end of his reign. He issued coins depicting himself as Mercury. Could this have been a cynical ploy to win over the Christians among Maxentius's troops?
Arguing against that, however, is that Christianity was far from a majority position in the West. Then, too, there was long resistance to military service among Christians. So the Christians among Maxentius's troops wouldn't have been all that numerous. As a ploy, it would be likelier to backfire.
In any event, Constantine's reign as the first Christian emperor was far more imperial than it was Christian. There were many divisions among Christians at the time, and the standard response was to read your opponent out of the Church. Since he would then read you out of the Church, the two of you would go on your merry ways in separate churches. Well, Constantine insisted on some sort of agreement, and he used the power of the empire to persecute the losing side.
I might point out that the late baptism was far from unique to Constantine. Christians of an earlier and of a later time would assume that any convert would be baptized immediately. In his time, baptism was regarded as a get-out-of-hell-free card. When you were baptized, that forgave all your prior sins. It didn't cover your subsequent sins. Thus, there was an argument for delaying baptism until you were about to die and didn't have any time left to commit more sins. Countervailing that was the observation that people, and Constantine ran more risks than those of his subjects who led quiet lives, could always die unexpectedly.