> But it was always completely obvious to me even as a child that the primary purpose of religion was to form local communities and have others with shared values to rely on.

That is an atheist point of view. I grew up in religious environment too and people really believed in god. Some were hypocrites and some values stand in a real life way (commitment to not lie makes for the worst business), but the basic believe that god exist was really real. And when you play the part just for social reasons, then I think you are just a hypocrite. Which would be good reason for people to leave - so that they dont have to pretend.