I would argue that "atheist" describes a person who would have historically chosen to align themselves with skeptics rather than something that likely would have gotten you killed. Who knows what the hell Socrates actually believed, but he was great at questioning the beliefs of his neighbors and was eventually executed for it. Frankly, I'm really doubting socrates would have frequented /r/atheism, very little of which is actually based around discussion. Nonetheless that's one of the earliest explicit references to not believing in gods as a point of morality or justice.

In this vein, you might want to look into pyrrhonism, which is basically an early, radical strategy against objective truth. The basic approach is to answer every answer with more doubt: it's quite easy with the help of the work of descartes & the rest of the renaissance and post-renaissance philosophers.

You might also look at the point where christian theology meets skepticism: maybe start with Aquinas. Especially because "atheism" == "blasphemy" at that point, you didn't see people outright claiming that god didn't exist, but rather using rationality as a tool to cut through religion-meets-xxx. For instance, you find Aquinas making arguments that churches and states should make sacrifices or compromises—i.e. choosing the lesser evil over a greater one.

Other forms of skepticism proposed alternative "theism" schemes to the predominant Catholic one. The beghards, Franciscans, Fraticelli, Minorites, Capuchins, etc., all challenged catholic dogma by making populist, skeptical, polemic arguments for, basically, choosing a different way to be pious, to find salvation, to combat indulgences, encourage reform, etc.

Martin Luther obviously kicked the discussion into high gear, leading to centuries of people acting crazy. Lots of skeptical arguments rejecting the Catholic church wholesale.

Finally, you begin to see philosophers arguing against religion, the christian god, gods in general, a definition for god, etc. At this point, atheism becomes an actual "thing" aside from "this dude disagrees with some fundamental parts of catholic teaching". Teaching and common arguments begin to reach the populace, and at this point I have some useful sources:

Name of the Rose. Frankly, a terrible historical reference, but one that is.... very good at portraying the transition from roughly the dark ages (please don't whack me) to roughly the renaissance, and patterns of thought we can easier understand.

To accompany the above, I propose Religion and the Decline of Magic, to continue the above line of thought on populist skeptic thought (and, in a somewhat amusing fashion, you see the catholic church scrambling to reform and formalize existing thought on witchcraft, ritual, language, etc. to stop their constituents from basically creating their own beliefs).

The Oxford Book of Skepticism. Basically a more general reference for "what was there to be skeptical about in this region at this time" and "what on earth does this movement actually mean in terms of belief". Particularly useful before start using the label "atheist".

More generally, I would advise you to reconsider what a "god" is. If the way you refer to "wind" is "bob the air god", it would be fairly perplexing to be confronted with wind and and then told "bob" doesn't exist. Just because people anthropomorphize "things" in the world that are not human does not mean they believe that they are conscious, or they have intention, or that they give a shit about humans at all. Without a way to objectively describe the universe, people have to resort to metaphor and anthropomorphize things to talk about them. When you're in physics class, you might think "ok, a nucleus of this size WANTS n electrons", but you might not believe that it really does have desire. Similarly, future atheists (or maybe even contemporary stupid ones) might get all in a huff because they think YOU think the universe is driven by electron gods.

So, it is really, really only recently when it even made sense to say you were an atheist and actually debate someone with strongly theist approach. Remember that until recently you would have been arguing about the inherent nature of the universe without any ability to even understand what "life" is, much less posit mathematically probable laws and models in a way you might be verified or not. Instead, you're probably looking for either counter-cultural, positivist, skeptic, or cult precursors to the modern friction between, umm, spirituality/education/logic/rationality/politics/morality/what-have-you and religion/anti-intellectualism.

EDIT2: Far, far more examples of "we believe not in objective truth but some other type of truthiness" appear in eastern philosophy (e.g. see koans + zen buddhism, flavors of gnosticism, possibly even mystery cults, though that is stretching 'eastern' to an extremely western viewpoint) as well as african philosophies (I know of a handful of examples in Igbo beliefs, for instance), but I am not very familiar with the literature or history of that enough to provide useful sources.

Abu Bakr al-Razi, a 9th century medical writer from modern-day Iran supposedly wrote two texts titled "On Prophecies" and "On the Tricks of False Prophets." In these he stated that human reason alone could give certain knowledge, the path of philosophy was open to all uses, that claims of revelation were false and religions were dangerous. This does not mean he was an atheist, but rather could just be a extreme reflection of the wider permeation of classical Hellenic (particularly Neo-Platonic and Aristotelian) thought and philosophy into the Islamic intellectual sphere, as many thinkers, such as Abu Nasir al-Farabi were attempting to integrate them into Islamic theology and philosophy.

There is written evidence for "unbelief," at least. John Arnold's 2005 book Belief and Unbelief in Medieval Europe reviews much of it. Arnold argues there are various levels of "unbelief," from heresies, to indifference, to possible atheism. I assume that last form is what you're most interested in, so let me quote a small sample from the book that reviews some of the documentary evidence regarding medieval "atheism":

"Around 1200, a prior of Holy Trinity, Aldgate wrote that 'there are many people who do not believe that God exists. They consider that the universe has always been as it is now and is ruled by chance rather than by Providence. Many people consider only what they can see and do not believe in good or bad angels, nor do they think that the human soul lives on after the death of the body.' Later that century, the Dominican preacher Remigio de' Girolami preached to Florentines that 'the fool, taking leave of his senses, obliviousness of the name of God, says in his heart "there is no God"' (Pss.14:1, 53:1). Other of his compatriots agreed that this was a problem with the laity of the city (though they may have been interpreting the implications of Florentine citizens' actions rather than reporting explicit unbeliefs)." (226)

Ultimately, Arnold's review of the evidence is somewhat speculative, with his interpretation often being posited as more of a thought-experiment than a conclusive argument. Much of the evidence he finds is more evidence of either a) laziness or indifference in keeping up with Catholic practices, such as attending mass, or b) heretical ideas regarding the nature of God, the universe, or the soul, such as the idea that the universe is eternal and God isn't. Neither of those practices is quite "atheistic." But the two sources in the above quotation are the most convincing that I can recall. The problem with them, however, is mentioned by Arnold himself in that parenthetical at the end: they might not actually be evidence of stated beliefs, but rather, clergymen's interpretation of the implications of beliefs or practices like the laziness or unorthodox ideas I just mentioned.

And my own related follow up for those able to answer this question... I recently read Doubt: A History, by Jennifer Michael Hecht which deals with the history of religious doubt starting with the pre-Socratics. How do those who've read it feel about her work?

Adding to this point, I believe I read something about some ancient Chinese philosophers' tendencies to be skeptical and atheistic. Can anyone elaborate on this?

In terms of heretical movements, there was never what would be considered to be an "atheist" sect. Heretic movements had a social program that also reinterpreted the religious tradition. It made more sense, for the peasant who had grown tireless of his conditions, to frame their demands for justice through demands for spiritual renewal and social justice as it already had been passed down to them by the clergy. So, at the root of things, people were more spurred towards a form of disbelief by thinking that god no longer spoke through the clergy, because of its greed, corruption and scandalous behavior.

Examples include such groups as the Cathars, who stand out as unique in the history of European social movements of the time for their abhorrence for war (including the Crusades), their condemnation of capital punishment, and their tolerance for other religions. They also rejected marriage and procreation and were strict vegetarians, both because they refused to kill animals and also because they wished to avoid any foods that resulted from sexual generation. Many Cathars did not follow these teachings, however, and scorned the importance that the Church assigned to chastity and even implied that sex was a sacrament. They preached that practicing sex was a way to achieve greater innocence, rather than it being a sinful act out of wedlock.

Also, there was the Bogomils, who refused any form of worship at all that was "born amidst the peasants, whose physical misery made conscious the wickedness of things." They preached that the visible world is the work of the devil and they refused to have children, in order to not bring new slaves into this "land of tribulations," as life on earth was called in one of their tracts.

So while these sexual creeds in no way infer a rejection of belief in god, they certainly are a challenge to the Church.