Now here's something worth thinking about...

If God is all-powerful, then the Devil must be nothing more than a darkness in the mind of God. But if the Devil is something real and separate, then perfection is impossible, and there can be no God... except for the aspirations of fallen angels...

(Taken from Mountain of Black Glass by Tad Williams [Otherland series].)
It's a bloody excellent read, by the way. I'm currently reading it for the second or third time - I love owning books, and thus being able to re-read at my lesuire!