It depends largely on your definition of atheism. If we include belief in any sort of some sort of "transcence"/"transcended plane" or "spiritual beings" of any kind as"theism", atheism was essentially non-existent in the historical record. You may get more stuff along th lines of "the gods don't care about us," but for a lot of people, believing in religion was like how we may believe in gravity or evolution. The idea of mental illness being a "sickness" rather than a spiritual affliction is a new one--it was sacralized before it was medicalized.

You can argue who the first atheist (or agnostic) to show up in the historical record is, how many of them there were or weren't, but it seems clear that they were at most small in number and generally from a thin elite. Now, lots of people accused their opponents of atheism, but this shouldn't be mistaken for a statement of fact any more than should the accusations of sexual and moral debravity against the same people. This accusations often took the form of laundry lists.

An important thing to realize, the philosopher Charles Taylor argues in his hugely influential A Secular Age, is that "the secular" is something added. It's not that you take away religion and you get secular society, but rather, to secular society is created out of religious society.

But, in short, atheism as we recognize it, and even likely agnosticism, would be vanishingly small if not non-existant in pretty much anytime before modernity. That does not mean people followed Orthodox beliefs, absolutely not, there were people who shirked religious obligations that they nominally believed in, there were people who held quite heterodox religious beliefs (my favorite is the Cheese and the Worms by Ginzburg; very readable for anyone with even a slight interest in religious history). The point is that rebellion to religious authority often took the form of alternative religiousness, rather than something like atheism.