It seems obvious that the ancient Israelites were not monotheists, but instead practiced monolatry, the worship of one god combined with a belief in existence of other gods. This is the best explanation of the first commandment's “no other gods before me” as well as of Psalm 86:8's “there is none like you among the gods.” But there are also events described in the Bible that suggest belief in other gods.
One that is particularly interesting is mentioned in the book Bible Prophecy by Tim Callahan:
In 2 Kings 3:27, when Moab is under siege by the Israelites, the king of that city sacrifices his firstborn son to the god Chemosh. As a result, “there came a great wrath upon Israel,” and Moab was spared. Now, why would there be a great wrath upon Israel if the only god in existence were Yahweh? It makes no sense for Yahweh to be angry with the Israelites for what the king of Moab did; if anything, he'd be mad at the king, and give Israel a victory. The obvious implication is that the god Chemosh, happy with the sacrifice, caused Israel's defeat. And yet this account was written by an Israelite, and included in their scriptures. This can only mean that the Israelites believed Chemosh was real. It also implies (even more shockingly) that they believed Chemosh had the power to do something that Yahweh didn't stop — and perhaps was unable to stop.
This is obviously a problem for any fundamentalist who interprets the Bible literally. But it is also a problem for every other modern-day Christian or Jew. Why would the one true God, who didn't want his chosen people to worship other deities, be okay with his followers believing these other deities existed? Worse, why would he allow them to suggest as much in their holy book? Isn't it far more likely that the Israelites were worshipping a false god (just like all the other ancients were), and that the monotheistic God that evolved from that false god is therefore also a fictional entity?