A bit of background first. We've been studying the Renaissance, and this question arose as we discussed Renaissance humanism, especially the more secular orientation of humanism in contrast to the very religious world view of the Middle Ages. I pointed out that even the most secular of humanists during the Renaissance were still believers, and this student asked, quite astutely "So when do we start getting actual atheists?"
It's an interesting question, if you unpack it. At what point does it become possible for someone to actively espouse the stance that there is no God?
My first impulse was to turn ot the ancient Greek philosophers. Plato and Aristotle both so depersonalize the transcendent as to create essentially atheistic world views, and the main Hellenistic world views - those of the Stoics, Epicureans, and Cynics - were equally, if not more devoid of a personal transcendent deity. But I have a hard time imagining that any of those fellows (with the possible exception of Diogenes) could have really functioned in classical society without at least going through the motions of the traditional religious practice demanded by their society. So, do they get credit for being atheists if they're still willing to kill a calf on Athena's altar on the proper day?
You can turn to some of the Chinese ethical systems, but there seems to me to be a similar overlay of traditional polytheisitic belief - followed even if not believed - there as well. And in the west things are pretty consistently theistic from the Romans through the Middle Ages.
The first actual philosopher who gets mentioned as a sort of atheist that I could think of is Spinoza - althogh he may be more of a deist.
So, I turn to you, learned denizens of the interactive Internet version 2.0, for your insights, and perhaps to spark a conversation. What's an atheist and when do they start cropping up? Or, perhaps to be more precise, when do we start seeing evidence of expressions of atheism, which is really all we can say. For all I know there were villages full of atheists in the medieval Pyranees, evidence of whom was so completely erased by the Inquisition that we will never know of them. But I'll leave that sort of speculation to the Dan Browns of the world.