I think that true, modern atheism like this would have been fairly uncommon in ancient Greece, though it certainly seems that Diagoras sounds like one. (One of my philosophy professors, himself an atheist, suspected that many of the great minds of ancient Greece were really closet atheists ... but I suppose there's just no way to know that.) Much more common than atheism was a re-definition of God in more abstract terms by Aristotle, the Stoics, and others.

Thanks for the link to Diagoras!