im an atheist but then again ive always been, way before weed or psychedelics [lsd and mushrooms just further confirmed my 'atheist creed']. If anything i think its the other way around. Possibly more atheists tend to gravitate toward weed because they dont have that central pillar of religion that their lifes revolve around. The idea of having a 'god' looking over and judging you might deter people from smoking weed. Im not sure though. I know plenty of people that are religous and smoke weed, I know the fiendiest of fiends that are devout catholic. I know atheists that smoke and trip.
I am pretty sure that your not going to get a clear answer though, thats just too vast of a generalization to make.