Diagoras

https://en.wikipedia.org/wiki/Diagoras_of_Melos

Diagoras "the Atheist" of Melos (Greek: Διαγόρας ὁ Μήλιος) was a Greek poet and sophist of the 5th century BC. 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.