"Lack of religion is either non-adaptive from a fitness perspective, or it is strongly tied to traits which are themselves non-adaptive."
I think you're missing that population control within available resource constraints is also part of the fitness equation. Also it's very likely that population expansion or contraction is more cyclical over the long term than a two year period of data would show.
Biology isn't my strong suit, so I hope that I'm not misunderstanding you, but wouldn't your statement imply selection at the group level?
Additionally, although the figures I quoted were over a two-year figure, they exemplify a larger trend. For example, the Wikipedia article on Australia's demographics show that Australia hasn't had replacement level fertility since 1975.
My statement really should have been worded as a question as biology, admittedly, it is not my strong suit either so I won't attempt to address your question but hope that someone with a better understanding will chime in here.
I think you're missing that population control within available resource constraints is also part of the fitness equation. Also it's very likely that population expansion or contraction is more cyclical over the long term than a two year period of data would show.