We are all aspects of God, whether we realize it or not. Ego is an illiusion, and is the opposite polarity of the Self. The struggle between the two is what we call the human experience.

God : Self :: Satan : Ego

Ego believes it can be Self, and tries to emulate it, but it is not and never will be, because it would be a lie. So choosing to follow the Ego leads to delusion, while following the Self leads to Truth.

Hope that makes sense. I'm no psychologist or theologian but this is how I've come to understand it.