When one is told there was a healthy atheist tradition (Charvaka, Ajivikas, Lokayata) in India, one must be careful. Atheist in the sense of not believing in rituals and the Vedas? Certainly. There are shlokas against rituals in the Bhagavad Gita too. Atheist in the sense of not believing in a creator God? Certainly. Atheist in the sense of not believing in the atman? We are on very shaky ground here.