In earlier (cursory) studies I had the impression that Constantine, "baptizing his regiments" had initiated the compulsory uniformity of the dogmas of Christianity. Today reading Hutchinson and Garrison's 20 Centuries of Christianity (1959), I've achieved a more complete view. His Edict of Milan, (sometimes called the Edict of Toleration) was remarkably progressive and liberal: "Liberty of worship shall not be denied to any, but the mind and will of every individual shall be free to manage divine affairs" (page 71). It wasn't until the American Revolution that this kind of toleration really became current.
Actually Constantine's religious toleration didn't last long; a few years after his death a later emperor attempted to "impose the death penalty on all who persisted in worshipping the old gods" (72). Theodosius I made "orthodox Christianity the sole and compulsory religion within the empire".
The authors conclude that rather than being a turning point in the history of civilization the edict became the "zero point" of a pendulum switching "from the persecution of orthodox Christians by a pagan state to the persecution of pagans and heretics by orthodox Christians in alliance with the state."
From that point until the 18th Century people were largely known by the religious label of their rulers-- a sad commentary on the essential nature of Christianity.