Constantine degreed Rome to be a christian.
Contrary to much popular belief, the Emperor Constantine wasn’t a Christian until his final days, he was a catechumen, a kind of adult, learner-Christian, and he wasn’t baptised until he was on his death bed. The Edict of Milan in 313 CE was actually an edict of religious tolerance, it didn’t suddenly convert the Roman Empire and Constantine himself maintained a relationship with late Roman paganism too. Throughout the whole 4th century, paganism continued in the Empire with various degrees of tolerance and outbreaks on intolerance and it wasn’t until the mid-5th century, with Rome already on the wane and Britain abandoned, that the harshest persecution of paganism began on a state level. This means that, especially in the more remote and outlying parts of the Empire, there was plenty of space and time for non-/pre-Christian beliefs to continue.