Like many others have said, the film is not necessarily about Christianity, and don't let the Christians who say otherwise convince you.
Throughout the whole movie, and particularly in the scene(s) where Cuba's character is first introducing Chris to the afterlife, it is REPEATEDLY stated that whatever a person WANTS their afterlife to be, that's what it becomes. I don't think that wasn't JUST a convenient plot point. In my opinion, it was also a direction to the VIEWERS of the movie that whatever form of spirituality THEY wanted to believe was being depicted, that's what it was.
When Chris asks Cuba's character, "Where's god in all this?" he replies,
"He's up there... somewhere. Shouting down that he loves us, wondering why we can't hear him. You think?"
This wasn't to specifically put the movie in a Christian context, but to recognize that there are people who do believe in a god, and in their own personal "heavens", he exists. Notice he added that little glimmer of doubt at the end, "You think?" right after the scene where it was pointed out that all you needed to do to materialize something in your world was to, essentially, believe it to be there and it would.
I actually think the line "[He's] shouting down that he loves us, wondering why we can't hear him" is a subtle and rather clever way of saying that the characters DON'T believe in a god (he's shouting, trying to get them to believe, but they choose not to so he thinks they just "can't hear him"). As someone else said, it was probably to keep the religious nuts from, excuse the pun, "raising hell" over a movie that doesn't fit with the very narrow belief system and considering the movie already lost a ton of movie without a boycott from the church, it was a good decision.
And by the way, obviously being an atheist myself, I believe the entire notion of an afterlife to be BS anyway, pretty much just a natural method humans find a comforting way of coming to terms with inevitable death. As someone who just doesn't find it necessary to believe in anything like that to continue functioning as a human being, I enjoyed the movie purely as a work of fantasy and nothing more. Anyone who thinks there is something wrong with that is, frankly, an ignorant moron in my book.