My comments on Rodney Stark’s Triumph of Christianity are what strikes me, and not an effort to be thorough.
Constantine’s combining of Church authority with State power was a mistake. It has hurt society and humanity.
Constantine was tolerant and cooperative with the pagans and other religions, yet he was intolerant of dissent within Christianity from Christian orthodoxy. His objection to dissent, and his application of state power against it was probably mostly trying to keep a strong unity, probably largely motivated by political ambition and avoidance of schism, which tends to lead to strife. (I don’t think Constantine was power-mad. I think he was sincere, but perhaps suffering from some noble-cause-corruption.)
I suppose Constantine was generally traditional.
That would mean that he expected people to honor their traditions whether they were different from his or not. It seems the Roman distaste for Christianity from the beginning was rooted in an expectation of following tradition and honoring the beliefs and gods of one’s family and heritage. Conversion to Christianity thwarted that. Conversion to Christianity abandoned one’s religious heritage. Traditionalists are likely to be incensed by such a change. Gradual change over generations was one thing. The dramatic conversion to Christ alone was seen as extreme, extremist, and antisocial.
For Constantine, with his Christian mother, Helena, he probably did not see his own conversion as abandoning his heritage and tradition. However, he probably respected such traditionalism among the pagans and other religions. He probably also tended to judge individuals by the content of their character, their abilities, and their political loyalty. He apparently continued always to honor and promote people around him without regard for their religious beliefs. He probably only considered whether or not they were reliable, and consistent behavior with regard to one’s beliefs, whether Christian or other, was evidence of conviction and reliability.
Regarding Constantine’s conversion, I suspect he was raised consistent with general Roman pagan tradition and beliefs. He probably had significant influence from his mother with regard to Christianity, but as a likely ruler of Rome, Roman religious practice was probably his own before conversion.
If one runs the numbers, given reasonable and plausible mathematical models (as Stark does in the book), one realizes that the Christians, who had been feared as potentially adverse political opponents, were at least a large minority, and probably already a majority, especially in the aristocracy. Constantine probably was mostly an opportunist. He saw the trend of increasing Christian unity and population proportion, and he decided it was time to embrace his mother’s faith. I think he was sincere, but I am nearly certain he saw only advantages for himself politically. Emperors were often assassinated by troops or guards. Applying Christian ethics in his administration and military leadership was very likely to improve his chances of staying in good graces with his subordinates and bodyguards.
I close this comment by reiterating that I consider the use of state power with any regard to religion a mistake and inherently wrong.