As usual, this is published a week after it premieres on my Tumblr (also titled The Ithildin Goddess), and includes some minor edits and additions.
This was a question directed at my blog some time ago and I am finally getting around to answering it. Sorry about the delay.
Zero. Zip. Zilch. Nada.
In fact, the very existence of the First Commandment outright nixes the idea that there is only one.
By saying “Thou shalt have no other gods before Me” YHWH was indicating that for those of the lineage of Adam and Abraham, he was the only God worth their time and devotion, as he was also the only one who would protect them — and Biblical history definitely bore out the latter point at the very least. He wouldn’t need to make this demand at all unless there were other very real deities with similar powers who might wind up being worshiped by the Hebrews, even after accounting for the obvious poetry about “money” or “the nation” or “the law” being false gods as well.
However, if you walked up to YHWH and asked if there were other gods, the answer would be an undeniable “yes”.
He wasn’t claiming to be the only god (not in the Old Testament at least) but rather the only God for his chosen people, the Hebrews.
And other tribes and civilizations record having been visited by their own gods with impunity. Ur, Babylon, the Norse, Scottish Picts, Aztecs, Incas, etc; most civilizations have been visited at some point by their guardian/patron deities. The list goes on and on.
In addition to these fine points, atheists will (wisely) point to a lack of theoretical evidence (being, evidence that can be used to empirically test a theory) that supports the existence of any gods, be they one or several. In short, god is presently unable to graduate from the “shaky hypothesis” stage of the scientific method.
A lack of evidence for any is also a lack of evidence for one, and this gives us, as stated in my opening, exactly ZERO evidence that there is only one god.
I suppose the next question you should be asking is “Does the lack of evidence matter to you, personally?”