The question is at least as old as the Book of Esther. When religious people write books that do not explicitly mention God or religious beliefs, but are written within a framework of faith, are those books religious books? Does Esther belong in the Bible?
What about J.R.R. Tolkien's fantasies, The Hobbit and the Rings trilogy? They're set in an imaginary world; the characters aren't really human; there's no mention of Christ or Christianity in these books. There's no controversial (some would say inadvertently blasphemous) attempt to imagine how other worlds might fit into the Christian belief system, as in C.S. Lewis's fantasy fiction, either. Nevertheless, Tolkien was a Christian, and as such he could hardly have written a story that didn't either affirm or reject Christian themes.
Billy Hallowell makes the case that The Hobbit affirms Christian beliefs: