Justanovel,
I must disagree with you, especially regarding, Lewis, whose Narnia books are not merely about good and evil but a clear allegory of Christianity's deepest principles. Unlike any other book I can recall reading, Lewis builds up an entire world with a history parallel our own, in a spiritual sense. And he does so with a keen eye to detail, including sin, the crucifixion, the destruction of the temple, the resurrection, the final battle, heaven.
In general though, I believe it is difficult to portray Christianity so clearly within a fantasy novel, if you are not earth bound. As has previously been mentioned, there is no reason to assume another world would so closely follow the trajectory of our own, on any level, including the spiritual.