But you, stand here by me, and I will tell you the whole commandment and the statutes and the rules that you shall teach them, that they may do them in the land that I am giving them to possess.
Deuteronomy 5:31

Men have always needed a mediator between them and God. Our flesh can't survive a direct, full encounter with him. Voltaire said, If God didn't exist, we'd have to invent him. I think he was right in some ways, but it's also true that if we don't know the one true Mediator between God and Man (it's not Moses, either), we are prone to invent false ones. See the golden calf and the virgin mary.