He lived most of his life as a pagan in a largely pagan soceity that regularly persecuted Christians. Of all of the religions floating around why would Christianity become the official religion of the empire? Was it fear of backlash if he chose something else, was it to promote tolerance and abate conflict, or is strictly utilitarian and Christianity is one the better religions for uniting people under a common rule?
There is an unwarranted cynicism in this post. Constantine’s mother Helena was a Christian and so he was exposed to the religion from an early age and was probably more sympathetic to Christians than his predecessors on that account, though he was publicly an adherent of Roman paganism in his early years. Eusebius of Caesarea is the primary source for the famous story of his vision at the battle of Milvian Bridge in which he saw a cross in the sun and heard the words “in this sign you will conquer” and although we will never know whether he truly had a religious experience we do know from independent sources that on his return from the battle he ignored the altar to the Roman gods at Capitoline and did not participate in the traditional celebratory sacrifices that were performed after a victorious return from battle, suggesting some change in how he regarded the Roman religion that happened some time between going into battle and returning, and the next year he decreed the Edict of Milan which made the empire religiously neutral and gave official toleration to Christians. At this point Christians were a growing minority, especially in the Eastern and Greek speaking parts of the empire but the Roman elite was almost entirely pagan and imperial power was not seriously threatened by Christians.
Though some suggest that Constantine’s conversion was wholly political, I can find no serious indication of that nor any concrete benefits that were to be had by him for converting. That is not to say that Constantine did not use his Christianity for political gain, bringing the church wholly under imperial control, as he was an astute politician, but it is my view from the facts available that his conversion to Christianity was earnest.
Before the Edict of Milan Christianity was officially illegal in the empire, but the worst of the persecution had ceased in 311 with the Emperor Galerius’ edict restoring the property that had been seized during Diocletian’s persecutions and granting Christianity the status of religio licita, the same status of Judaism, wherein the belief was recognized and tolerated without receiving official approval. Christianity did not become the official religion of the Roman Empire until Emperor Theodosius’ Edict of Thessalonica in 380.