[QUOTE=RickJay]
I hate to be a jackass, and this will undoubtedly cause me to make a variety of comical typos, but it’s “atheism.”
To directly answer your question, it sort of depends on your definition. There was unquestionably a time when human beings existed, probably very early in our history, but had not quite gotten to the level of tribal organization and shared lore and history that would have led to a belief in theistic beings.
Non-theistic belief systems go back at least as far as the beginnings of Buddhism, Taoism, and other such systems that are, at least technically, NOT based on belief in gods.
In Western civilization, Epicureas was an atheist; he openly didn’t believe in the pantheon of Greek/Roman gods, or at least didn’t believe they WERe gods, but rather were mortal beings. He was born somewhere in the vicinity of 340 BC, so that is probably the best answer to your question.
Once we get rolling with Christianity, though, your general impression is probably correct at least in the Western world; virtually everyone believed in some sort of god or gods, and if anyone came to doubt them they likely would have kept their yaps shut. There have always been people who were indifferent to the whole idea, but outright atheism, an active belief that there is NO such thing as a god, probably does not predate the Enlightenment as an openly held beleif in the Western world. In the East, as I said, there have long been “religious” traditions that did not necessitate belief in a God.
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As far as Buddhism goes, my impression was that it was not particularly “atheistic”. It is true that some forms of philosophical Buddhism don’t require any gods for salvation, but that isn’t the same thing as denying they exist - indeed I seem to recall that some strains of Buddhism consider that ‘being a god’ was somewhat unfortunate, as it was only humans who have the opportunity for nirvana.
Moreover, in many forms of Buddhism various supernatural beings that act more or less like saints and gods show up. Same in Taoism.
Taoism is like Buddhism in that philosophical Taoism has no particular need of gods or other supernatural entities.
So far, Epicureas seems the best bet; what about Confucious? Any ancient Egyptian expressions of non-belief? I suppose earlier expressions are unlikely, as it was mainly the priesthood who could write.