The ancient Greek poet/philosopher Diagoras of Melos (5th Century B.C.E.) might be regarded as history's earliest known confirmed Atheist. Historical evidence shows that Diagoras wasn't afraid to speak his mind regarding his atheism.
Though information on Diagoras is somewhat scanty, he was documented by Aristophanes, Cicero, and others. In fact, Diagoras' giving offense to others by rejecting "the Gods" is the one point about him that historians can be certain on. According to Athenagoras, "he made the downright assertion that god does not exist at all."
Diagoras appears to have been very bold, witty, and unapologetic about his lack of belief in deities. From historian Jennifer Michael Hecht:
"The poet Diagoras of Melos was perhaps the most famous atheist of the fifth century. Although he did not write about atheism, anecdotes about his unbelief suggest he was self-confident, almost teasing, and very public. He revealed the secret rituals of the Eleusinian mystery religion to everyone and "thus made them ordinary," that is, he purposefully demystified a cherished secret rite, apparently to provoke his contemporaries into thought. In another famous story, a friend pointed out an expensive display of votive gifts and said, "You think the gods have no care for man? Why, you can see from all these votive pictures here how many people have escaped the fury of storms at sea by praying to the gods who have brought them safe to harbor." To which Diagoras replied, "Yes, indeed, but where are the pictures of all those who suffered shipwreck and perished in the waves?" A good question. Diagoras was indicted for profaning the mysteries, but escaped."
According to the Swedish historian Stefan Stenuud, Diagoras was expelled from Athens in 411 B.C.E. for attacking religion. Stenuud also documents events that may have led to Diagoras' atheism:
"According to Sextus Empiricus, Diagoras became an atheist when an enemy of his perjured himself in court and got away with it. There are some variations in other sources to this anecdote, though not changing its moral content – immorality seems to go unpunished, so how can there be any gods in the sense of watchers over human virtue?"
None of Diagoras' writings seem to have survived, unfortunately for us infidels...
Mark Tiborsky, the Cleveland Freethinkers