I could see if the paganization was only that a religion based its beliefs on love, kindness and good morals that are helpful for the self or other humans. But to be a "Me too" religion replete with virgin births (on the same day as another God Mithra), miracles, a resurrection should leave someone to question the authenticity of that belief system.
If I came down from some other planet and was learning the belief systems of humans and had to decide which ones were true, if any, and you told me of Christianity, it would be the least likely belief system of all that had any semblance of truth. It would tell me that 2,000 years ago people were hyper religious, believed in apparitions, believed in vengeful Gods that could alter time, mathematics and reason, claimed that a very small tribe was his favorite, that blood sacrifices of animals and humans were pleasing to him and to prove it, sacrificed himself to himself for 3 days to save us except we weren't, did miracles to impress people and cured only a few people but not everybody, I would immediately leave the planet thinking the inhabitants were still too easily persuaded by stories.
In the time of Constantine, the region was plagued by wars. He never was a Christian. He was a powerful man and told the high priests to get together and get their act together to decide on which books should stay or go, what their dogma would be and to make sure their God was an invisible being who monitored everything a human did including in the bedrooms and if they crossed Him, killed anybody, stole things that He, (God) would deal with them. Constantine knew that their God would be the Cop in the sky and alleviate his problems with a warring populace.
Constantine's conversion to Christianity also helped to bring stability and unity to the empire, which had been divided and plagued by internal conflict. By promoting a common belief system, he was able to bring people together under a shared set of values and ideals, which helped to strengthen the empire as a whole. Thus, just like it is today, the leaders indoctrinate a gullible populace (as we see with the TV anchor's montages given them by the Demoncrats) and control the populace so they can stay in power or gain power, just as the Dems used lawfare to bring down Trump.