Pagans have been around way before monotheisim granted. But the Catholic Church converted most pagans by adopting some of their ways of worship. The biggest example of this is when they changed the sabath(day of worship) from Saturday (friday sundown till saturday sundown like the jewish) to Sunday. Also Christmas. and this became the norm in christianity once the Catholic church became prominent again.
Edit - Side Facts
Christianity is derived from Judiasm, because Jesus was the "king of jews". And since christianity is supposed to be the following of the path/life Jesus lived, It would be resonable to say christians are supposed to pretty much do everything the jews do except they believe that Jesus is Gods son and The Savior. So saying that Christianity takes from other religions is a lie. Christianity is born from Judiasm and since Judism/Hebrew is the First religion to believe in One God (Monotheism), you can pretty much say that it's one and only influence is Original and One of a Kind.
Heres a link on Judiasm