The problem of Evil
The figure of Satan as progenitor of sin poses logical and ethical contradictions within a monotheistic religious system that denies the ambivalence of the creator god.
As far as the Christian Dogma is concerned, the implicit dualism of the existence of Satan has never been truly reconciled with the Canon.
The Lord was infinitely good; evil had its source outside of him. Yet he was the author of all things, and ultimately responsible for his creation. How could he allow evil to exist?