Speaking as a solitary practitioner here- not believing in religion does not automatically make one an atheist.
Religion is a highly personal thing, and not everyone can find a comfortable mindframe within community efforts for how they percieve deity, or lack thereof, within the world.
One should also consider the stance of agnostics.
Agnostics being people who believe that there may be a higher being/s, but not interested in following an organized form of religion, as opposed to the atheist who, by definition, refuses, or are unable, to believe in the existence of a higher being or beings.

(Although I must admit, I am something of a convert to the Church of FSM.)