"Atheism" as it's currently defined and thought of is a very modern worldview and would be anachronistic. Would the Epicureans be close enough for you? Epicurus (3rd century BC) and his followers rejected the idea that any gods that may or may not exist intervened or were even really aware of the mortal plane and that the material universe was formed by different combinations of tiny particles named "atoms."

The most famous piece of Epicurean literature was De Rerum Natura or On The Nature of Things by the Roman poet Lucretius in the 1st century BC, and Lucretius actively rejects the idea that the gods have any real role to play in the functioning of the physical universe. He does not, however, rule out the possible existence of the gods.

http://classics.mit.edu/Carus/nature_things.html

Atheistic physicalism/materialism is mentioned in the Buddhist Suttas, around 500 BCE (transmitted orally for a while, and only being set to print around 29 BCE or so?). It's called "Ucchedavada" in the Suttas (you can search for "Annihilationism" because it's a long Sutta). As near as I can tell atheism is as ancient as any religious doctrine. People have been predisposed to equating consciousness with the body for a long time if we trust history.

If by "theism" you mean an acknowledgement of an Abrahamic conception of God, then Buddhism may be regarded as atheistic in and of itself, although Buddhism is non-physicalistic. So in Buddhism ideas like rebirth and volition make sense, whereas in materialism and physicalism they generally do not.