[wbarmy]

Many, no doubt, look for political reasons behind Constantine’s shift from pagan piety to Christian devotion. But frankly, the political explanations make very little sense. Why would an emperor motivated purely by power, abandon the religious practices favored by an estimated 80-90% of Roman citizens in AD 312 to take up the banner of a small and despised sect which, as recently as six years before was subject to the harshest penalties of Roman law?

I think that in fact, the 80-90% of Roman citizens being pagan was manifestly incorrect. There are good studies which show that as much as 50% of the population had become Christian and that this was having an effect on the military. Think 40 martyrs of Sevaste.

[Antoninus]

There are good studies which show that as much as 50% of the population had become Christian and that this was having an effect on the military.

You'll have to provide some sources for this claim. I have never seen anything like the empire being 50% Christian in the early 4th century in any serious scholarly work. Maybe by the end of the 4th century, but during the Great Persecution or early in Constantine's reign, no way.

That said, there were definitely Christian units in the army from at least a century before Constantine:

When rain saved the Thundering Legion ~ Legio XII ~ during the Macromannic Wars of Marcus Aurelius