|Diagoras of Melos|
A 5th century BC Greek poet and sophist.
"Throughout Antiquity he was regarded as an atheist. With the exception of this one point, there is little information concerning his life and beliefs. He spoke out against the Greek religion, and criticized the Eleusinian Mysteries. The Athenians accused him of impiety, and he was forced to flee the city. He died in Corinth." (Wikipedia)
1. According to Tim Whitmarsh in Battling the Gods, he is "the first person in history to self-identify in a positive way as an atheist."
Note that Whitmarsh goes on to say that: "Atheism, in my opinion … is demonstrably at least as old as the monotheistic religions of Abraham." (i.e. it predates Christianity and Islam)