Mildly Interesting

While I have not lived in the south I do have some deeply religious and conservative family members, so I do see where you're coming from. I just think even a good chunk of the people you mentioned have an affiliation to fit in and not because they are genuinely religious, i.e. pray regularly and go to church every Sunday. In other words, it's a cultural thing. They probably wouldn't go so far as to consider themselves atheist, but I could easily see them considering themselves culturally Christian and non-religious in practice. I have no idea what percentage that actually breaks down to, but my guess is it's a decent amount lower than 70%.