Part 17: Night10194 goes into how Christianity adopted things into its religion.
There's also the fact that Constantine basically co-opted his own pagan faith into early Christianity. This is the reason why Catholocism seems more interested (or at least focuses more heavily on) the new testament and Christ the Son of God rather than the old testament and the God of Abraham. It's also the reason why Christ, the Angels, and the Saints are always depicted with halos of loght: his pagan faith worshipped the Sun as God.
(And said god was a man who died and reincarnated as the Sun God. Sound familiar?)
It's partly this, and partly that a religion with an active and effective missionary tradition will pull this kind of stuff everywhere it goes. "Oh, you have a hero? Yeah! Uh, our God is totally okay with an awesome guy like that. Yeah, he's been a Saint all along. You can keep worshipping him, just add these bits." and other means of finding points of cultural contact are basically how effective missionaries do. Also, a lot of the New Testament vs. Old Testament stuff was actually from the Apostle Paul, who is probably one of the most successful missionaries of all time, spreading the faith through 'god-fearers', people who accepted the basic principles of Judaism or were exposed to it by their neighbors among the Helenistic Jews but couldn't join because it was partly a matter of the Chosen People. If you really want to spread your religion, either find people who already half understand what you're talking about or figure out ways to make what you're talking about fit what people already understand.