It's a mix bag of a lot of different stuff.
LIke the Gospel of John is a swipe of Philo of Alexandria's import into Judaism of Plato's logos with a heaping helping of the Cult of Dionysus. John is the only description where Christ does the water into wine thing, and there are a lot of stories about Dionysian doing the same trick well before Christ walked the earth. Also, the whole Jesus is the true vine thing. Oh, and the story of adult Jesus and the The Bacchae have a lot of similarities. Specifically, an incarnate god comes to the land where he is worshiped, but isn't recognized because he's human now and gives vague answers about his divinity.
Then the gospel of Luke that has the only account of Jesus's childhood. Gods in the ancient world always started out as babies with miracle powers. That's how you knew they weren't a regular kid. It also was written 200 years after the birth of Christ, and was likely meant to convert pagans via the child miracle stories.
Then, of course, there's the whole cult of mythras. There's an entire grad course worth of documentation on how similar Christ is to Mythras.
And Saturnalia. Or as we call it, Christmas. It was celebrated on Dec 25th. Along with a rival cult Sol Invictus's birthday being on the same day.
On the whole, Christianity is just a bag of "oh, we do that too but it's called..." as a religion.