Maybe he saw the idea on the skins of another tribe in the area.Here is where the theory of God falls apart, "where did God come from".
Intelligence. Abraham started off as an atheist, smashing statues of gods, and dismissing ignorant superstitions.
It wasn't until he began to reason intelligently about what the nature of God as a living being must be that the living God appeared to him through dreams and visions...
So based on that one can surmise that God came from an incorporeal realm of conscious life somehow connected to the only thing incorporeal in human beings, the conscious mind.