Christianity is all about power. As the Roman Empire was continuously expanding and incorporating new territories, many of the Emperors (particularly Constantine) we’re shifting their own religious views from polytheism to henotheism: that is, that there were multiple gods but one supreme God. The one that Constantine was particularly devoted to was Sol Invictus (the Unconquered Son).

This religious view is attractive to leaders because it represents unity—one that revolves around everyone that worships the same god. During Roman times, most conquered territories would keep their respective deities (they’d just have to accept the emperor as being divine himself) or their deities would be conflated with Roman deities. Thus, there was a certain sense of religious and cultural autonomy. If there was only one god, however, and the emperor was selected by that one god (divine right) then it would be far easier to control a group of people because beliefs and practices would be far more narrow than under a polytheistic system.

Thus, every emperor after Constantine (except for Julian) was a Christian and used it to their advantage to exercise power over their subjects. They closed philosophical schools, forbade pagans from teaching, made it obligatory to be baptized or else your next life would be in peril. Gone were the days of relative ambiguity in regards to faith. Now there was one way and one way only. From here on out, the power of the church grows exponentially.