So in the hoorow leading up to the release of the final book in the series, someone asked Rowling about her religious beliefs. She said she preferred not to discuss them, not because it made her uncomfortable, but because it might give away crucial plot points in Harry's story. At which point I turned to ... well, someone, and said, "Ah! She's a Christian, and Harry is going to sacrifice his life and resurrect, which will somehow redeem the Wizarding World."
Which is exactly what happens in the last few chapters of
Deathly Hallows. (I also predicted who would play Horace Slughorn, but that's not important right now.)
Further, in the churchyard scene referenced by
@His Dewliness, the gravestones for both the Potter and Dumbledore families are engraved with explicit Biblical quotations, though they aren't identified as such. This, along with the fact
every single book (not sure about the movies) includes an important scene at Christmastime, establishes pretty thoroughly that, yeah, Christianity exists in the Potterverse.