Under Polytheism, people could have a god for every different aspect of existence: but when, under Monotheism, people tried to unite all gods into one, there were some dualities they could not transcend, so that they had to choose one that would become their God, and one that would become its Satan.
Unlike Oriental religions, no Occidental religion could not unite mind and matter, and as religion is itself more mental than material, it came to see the body as the domain of evil and the mind as the domain of good.
As instinct is concerned mostly with the physical and analysis with the mental, monotheism became a repression of the id, symbolized by the Devil or “Beast,” by the ego, symbolized by the Lord or “Father”, which had earlier been balanced as Dionysus and Apollo.
Heaven and hell are a symbol of our unconscious mind, where our emotions are turned into angels through acceptance, and into demons through repression. Therefore, the more we repressed our emotions, the more afraid we became of them, and once we expressed them they were, at first, excessive.
Aside from Christianity and Satanism, the duality between emotion and reason would later take on two more major forms in the West: Romanticism and Rationalism, and Counterculturalism and Modernism.