There existed in ancient India an atheistic school of philosophy. The proponents of that school were known as Charwaks or Lokayats. Charwaks refused to believe in God, rejected the notions of Heaven and Hell (as well as of Rebirth and Nirvana), dismissed the idea of Soul being some entity different from Body, asserted that one's true salvation lay in being able to enjoy freedom, criticized the servile fidelity demanded of women by men, and regarded any social discrimination on the basis of caste or color to be unacceptable.
Charwaks ultimately lost to the religious masses around them, but their legacy never completely died out. My first (and only) education in the history and philosophy of Charwaks resulted from reading a scholarly treatise in Marathi written by my father Sadashiv Athavale (1923-2001).
That treatise has now been translated into English by Suman Oak and is available as CHARWAK: HISTORY AND PHILOSOPHY. This link is pertinent.