This is hard to figure out. On the one hand, it would not make sense for him to come out in favor of Christianity to gain the support of Christians. When he did this his main supporters were pagans who despised the Christians. Christians made up a small minority and mostly belonged to the poor lower classes with little to contribute to his cause. This seems to favor his conversion being genuine.
While on the other hand, his “conversion” did not stop him from believing in other gods. Throughout his career he seems to have thought that the Unconquered Sun god and the Christian God were the same, two views of the same God. The other gods were still real and powerful to him, and he would at times consult the oracle of Apollo. He accepted the title of High Priest of paganism and led in all kinds of pagan ceremonies with no concern that he was betraying the God who had given him victory.
The evidence seems to say that Constantine was a “Christian” outwardly, but not inwardly. In other words he went along with the outward trappings of Christianity but not the inward reality.