I've been listening to a lecture series on ancient Near Eastern religions of late, and the lecturer has confirmed something I've suspected for some time: the ancient Israelites, at least up through the time of Moses, were not monotheists, but, rather, henotheists -- that is, they did not believe that there weren't many gods, but that they had formed a covenant with one of the many gods, and were obliged to worship only him:
"I am the Lord your God, who brought you out of the land of Egypt, out of the house of slavery;
"Do not have any other gods before me...
"You shall not bow down to them or worship them; for I the Lord your God am a jealous God, punishing children for the iniquity of parents, to the third and the fourth generation of those who reject me..."
There are other gods, but the Lord is "a jealous God" -- not the only true god, mind you! -- and resents other gods being worshiped.
UPDATE: I reached the lecture today where the lecturer concludes that true monotheism developed in Israel around the time of the prophets.