Nothing. It is just an excuse for the psychologically maladapted to assert control. Furthermore, I have found that the people that have issues with polytheism are often the largest threats to civilized thought. These folks are so neurotic, that they cannot understand anything about God that isn't in their narrow field of view. This neurosis makes them even falsely label our ancestors as "polytheist" and all kinds of nonsense. Even if they were, so what? It should tell you how weak someone's god is if they must evangelize the whole world in order for it to exist. God as I know it simply is whether I tell you about it or not.