Lets try this one on.
If you believe in God, you must believe in the Devil. Believe in Angels, and in Demons. You can't have one without the other.
So how can you have this ultimate good AND ultimate evil? Either someone created them to offset each other, or one created the other.
If both just "always existed" that means that there are more than just one omnipotent "god" and perhaps more than just the two at hand...
If God created the Devil, that means God is the source of all Evil in the world. Or if the Devil was accidently created, that means that god can fail...