Many of the Greek philosophers in the sixth through fourth centuries questioned the myths of the gods, but I get the sense that they all (even Xenophanes) believed in gods or a god of some sort -- they didn't argue against the existence of divine being(s) in general.

See, that's what I was thinking, too. The idea of men creating the image of gods as men seems to go hand in hand with Classical Greek art. But in a different conversation, a friend pointed out a figure called Diagoras the Atheist who was a contemporary of Xenophanes (or slightly after). Wikipedia has a link to his philosophy, (http://en.wikipedia.org/wiki/Diagoras_of_Melos#Philosophy) and it certainly looks like the root of modern atheism.