... an event that happened only in fictional "histories".
Tibi gratias. The_Reader_David puts it a bit more smoothly: Constantine didn’t make Christianity the state religion of Rome, just made it legal (and gave it a boost up by having a newly built Christian city — in the sense it had lots of churches and no pagan temples — made the new capital). The Empire didn’t become formally a Christian state until decrees of Theodosius I in 380.
I apologize for over vividly remembering the story of the Milvian Bridge. But I really do not apologize for reminding people that Christianity ... a Divine Intervention in Human History ... did not eliminate it. Furthermore, the pagans did not go all that quietly, and had a revival or two of their own along the way.
Personally, I have always been rather proud of the church's connection to the classical world. To deny that the Roman Catholic Church is Roman, or that the Greek Orthodox Church is Greek, and that both subsumed elements of pre-Christian practice strikes me as the height of religious prudery.
Perhaps it is an over-reaction to those Catholic-hating early Protestant divines who sought to remove every practice they considered "pagan" from their new churches. For example, the Puritan leaders of the Bay Colony made the celebration of Christmas a crime.