Firstly we need to understand that most people before the contemporary era that are called atheists normally wont identify as so, they still have ideological belief structures based on faith, so these can be only classified as Deists, Monists etc.
Atheism as concept in pure form cant really work in a society devoid of the ideological presuppositions of the contemporary era, this can also be observed when we discuss a much more recent phenomena, the proper reality of the period and the lacking nature of the universe without ideological justification would any atheist seen ludicrous, because fundamentally their concept of the universe is lacking pieces that we could only discover later, thusly atheism was mainly used as a tool of shame in public settings for people that proposed different ideological forms of contact with higher truths.
My sources on these issues are At the Origins of Modern Atheism, by Michael J. Buckley, reportings on the life and times of Socrates and Pythagoras, and an essay about science and other types of belief systems for which the name escapes me now, it should be somewhere in my notes, I will come back to edit this later when I find it.