A Short History of Freethought, Ancient and Modern, Volume II

"It was about that time [415 BC] that the poet Diagoras of Melos was proscribed for atheism, he having declared that the non-punishment of a certain act of iniquity proved that there were no gods. It has been surmised, with some reason, that the iniquity in question was the slaughter of the Melians by the Athenians in 416 BCE, and the Athenian resentment in that case was personal and political rather than religious. For some time after 415 the Athenian courts made strenuous efforts to punis...