To be clear, Atheists have different belief systems. They all have in common that they believe in one less god than monotheists, and a handfull less gods than polytheists (that, they share with monotheists), but overall you can have different belief systems that do not involve the existence of one (or multiple) god(s).
I know someone who does not believe in gods because he think aliens are playing with us (fake flags etc. Basically the Stargate mythos, except he never saw the show). Less anecdotal, Chalmers is definitely Atheist, but i sure don't have his belief system, and i'm sure no functionalist does.
Also, in general, philosophy of mind is imho the best way to test you belief systems. Or maybe it's the philosophy field i'm the most comfortable in and thus the one where i pushed my beliefs the furthest :/