I don't believe that it is possible not to believe in God. It is possible to believe that the Christians, Jews, Muslims or Hindus are wrong in their beliefs about God, but not that God does not exist. God does not have to be a supernatural creature. It can be, for example, the universe itself, the rules of physics, the space-time continuum, etc. Whatever you believe to be the basic "root cause" of things existing rather than not existing is God.

To deny the existence of God would be to deny the existence of anything at all, including your own mind.