Why did Constantine convert to Christianity?

1. First, there’s the context, which is to say the persecution of Diocletian. Diocletian’s persecution was the first top-down, empire-wide systematic persecution of Christians, who at that time were an unpopular religious minority comprising maybe about ten percent of the population of the Empire. In hindsight, we can say that Diocletian’s persecution of Christianity was pretty clearly a failure. The question here is, what would ordinary people at the time have thought of it? There’s no evidence, but we can try to fill in based on psychology. Roman paganism was not a religion where belief was important or necessary. A Roman pagan might look askance at Christian emphasis on belief, but when faced with an environment where a significant number of Christians were dying, willingly, without putting up resistance, for those beliefs, well, martyrdom is a pretty powerful propaganda tool. It’s certainly possible that Constantine, as a politically minded individual, might have seen this oppressed minority as an opportunity. One can also see that the vision of Constantine does not exactly accord, in a doctrinal sense, with pre-Constantine Christian thought. Christian doctrine did not lend itself particularly well to a narrative of military conquest, and was, in books such as the Apocalypse of John, pretty strongly anti-Roman. None of this, however, has necessarily any bearing on whether or not Constantine’s beliefs were genuine. Some of the “evidence” which is claimed to support the thesis that Constantine was insincere relies on a misunderstanding of the nature of Christian belief at that time. For instance, there is the matter of Constantine not being baptised until he was on his deathbed. This is not unusual in light of the fact that doctrine of the time held that forgiveness for one’s sins was a one-time-only event. Surely Constantine knew that as an emperor, he would have to do some pretty nasty things, and so it makes perfect sense to defer that forgiveness as long as possible. (You can see in this example why that doctrine changed.) As for the monuments that show pagan imagery aside Christian imagery, this again does not necessarily indicate that his beliefs were insincere. One finds this sort of syncretism extremely commonly in societies which have just adopted Christian beliefs, and it’s easy for us to judge, but Constantine was first and foremost a political leader, and had to work within the limits of his political power. In addition, as previously noted, he was not a baptised Christian for the vast majority of his life. To me the most interesting evidence that Constantine’s beliefs were genuine is his equivocation on the issue of Arianism. Constantine convened the Council of Nicaea, but took no part in its debates. When the council condemned Arianism, he accepted its judgment. Late in life, however, living in the East, he changed his mind and began to endorse Arian beliefs.
2. https://www.youtube.com/watch?v=9oYkJcTZUlE