One night Emperor Constantine, a Pagan woke from a dream. In that dream he had met the God of the Christians who told him he would assist him in battle against the east and would be victorious. Constantine granted free-worship for all, including Christians. He thought that this is what was needed to unify his empire. He could keep power over the Roman Empire and just in case Christianity were true, God would keep the pearly gates open for him. What did he have to lose?
"The Church had acquired a protector, but it had also acquired a master."—A.H.M.Jones, Historian
He ordered all statues of himself, to hold a sword in the shape of a cross.
In the end he thought himself a Sun-God, Pagan Chief Priest, Pontifex Maximus, supreme ruler over all the other religions. Christianity thus became a powerful political and religious entity, part Christian and part Pagan, e.g. using astrology and divination, things the bible condemns. Deuteronomy 18:10-12
Constantine became baptized a few day before he died in 337 C.E.
Christianity grew and grew, becoming more worldly, dividing into the many different denominations that it has become today.