I'd offer this idea in return -- religion may be the collective, social expression of individual spirituality, usually fuzzy and generalized beyond an individual's own specific beliefs in order to promote social harmony and cohesion.

Just as some people have "religious" beliefs that are very eclectic and isolated from organized religion, a sort of personal spirituality, I think there are atheists with particular and non-social views.

I also think many atheists, sharing similar "spirituality", find a similar kind of comfort in "communing" with other "believers", and socially expressing their beliefs with other "believers". Regardless of the specifics of said spirituality, I believe we do see some commonalities between "theists and atheists".