D.G. Myers makes the case:
Speaking as both an author and scholar of fantasy, [C.S.] Lewis said in a 1947 essay that "To construct plausible and moving ‘other worlds’ you must draw upon the only real ‘other world’ we know, that of the spirit." No statement about the genre has ever been more definitive. The bedrock premise of fantasy, which cannot be waived without voiding the genre, is the existence of a spirit realm. Lewis’s Narnia, Tolkien’s Middle Earth, Rowling’s "wizarding world," parallel universes of all kind are imaginative reconstructions of Christianity’s first principle: namely, that the "kingdom of heaven" is the only true world.
The Dish 