Between the Rivers — Harry Turtledove

At the sun-drenched dawn of human history, in the great plain between the two great rivers, are the cities of men. And each city is ruled by its god.

But the god of the city of Gibil is lazy and has let the men of his city develop the habit of thinking for themselves. Now the men of Gibil have begun to devise arithmetic, and commerce, and are sending expeditions to trade with other lands.

They're starting to think that perhaps men needn't always be subject to the whims of gods. This has the other gods worried.

And well they might be...because human cleverness, once awakened, isn't likely to be easily squelched.