They formed what has come to be known today as the Catholic Church. The new church made a deal with Rome [and the Devil] to become the State religion portion of what they labeled the Holy Roman Empire.
The church got protection and then power from the Roman State; and Rome got the allegiance and respect of the Old Churches followers, to bolster government power and authority. That apostate Church then hid the Bible from mankind by forbidding it to exist in any except the dead language of Latin, which only her specially trained priesthood could read. At the point of a Roman spear, then, Catholicism became the only and world wide religion. For over a thousand years, during what has come to be known as the Dark Ages and with her influence persisting to our day, Catholicism ruled the earth by publicly torturing and doing their best to kill everyone who didn't join them.