I don't want to discourage further answers but while you wait there's been a few good threads on this in the past that might be of interest including:

Assuming an atheistic perspective, what factors gave rise to the widespread adoption of Christianity in ancient Rome prior to it being made the official religion of the empire? by /u/Zosim

How did Christianity go from a small, hated cult to the state religion of the Roman Empire? What made it so different than the hundreds of other small cults throughout the Empire? ,How significant was Constantine's conversion to Christianity? and Why did Christianity, and not another contemporaneous cult, spread so rapidly within the Roman Empire? by /u/talondearg

How did Constantine's adoption of Christianity change it? by a deleted user

Was there much resistance by Roman citizens to keep worshipping Roman deities in the post-Constantine Rome? by another deleted user

How late did Greco-Roman polytheism / paganism last as a religion after the Roman Empire became officially Christian, and where were the last areas it was practiced? which has a lot of good answers.