Atheists like to tell us benighted monotheists that they simply believe in one fewer god than we do. They wait for the puzzled neanderthal gape, which admittedly they sometimes get, and then explain, "Well, you deny all the gods of the ancient polytheisms for precisely the same reason we deny yours, to wit: they are just figments of imagination (and quite possibly some kind of neurosis as well). Well, then, why do you believe in just one God? What is your principle for accepting one God and denying all the others, eh? Aren't you being just a tad inconsistent?"
No, we're not. Aquinas actually gave this challenge a long, exhaustive answer in Chapter 42 of the First Book of Summa contra Gentiles. I will be a bit more brief: For monotheists God is not simply a supernatural entity with comic book powers, he is actually Being itself and as such the source of all being. Therefore, we monotheists believe that there can only be one God because there is only one principle of the to be. If there were multiple gods, there would be multiple principles of the to be. But everything in the universe at least has one thing in common, namely that it exists, it is a being, it has a to be. But multiple principles of existence mean no unity of being, and that would wipe out the commonality of existence. And so non-being would no longer be the only opposite of being, being could be as well. And this is clearly absurd.