Atheist means one who does not believe in the soul and God. These are correlative terms. If you understand what is soul, then you can understand what is God. If you understand what is God, then you can understand what is soul. But those who are agnostic, atheist, they neither believe in God nor in the soul.
Lecture on Bhagavad-gita 2.26-27, London, August 29, 1973