Without ceremony, religion is empty.
Without children, it’s pointless.
Without a strong sense of ethnicity, religious identity disappears.
People go to church because they have kids. It happens almost mechanistically. People go to church when they are little kids, because their parents force them to. Then they move out of the house and stop attending–even the devout ones. A decade later or so, they have kids of their own and start feeling the yearn for some sort of religion in their childrens’ lives–something to teach their kids the ethical norms, values, and traditions of their culture–and so they head back to the church of their childhood.
Remember, religion isn’t just a bunch of factual statements about god. If it were that, there wouldn’t be a lot to say. It’s not something people logically believe, because if they did, the children of Muslims in Pakistan would be just as likely to turn out Christian as Muslim. Religion is about culture/ethnicity–it’s a specific subset of culture/ethnicity that happens to sometimes involve a deity. That is, Pakistanis like Allah just like Finns like Deathmetal.
People want to teach their kids to be good people, to be good members of their communities and follow the values of their ancestors, and this is where religion comes in. People go to church specifically for the purpose of getting someone else to spend half an hour trying to
cram civilization into their kids teach their children morality, culture, etc. Heck, we even invent extra deities (Santa Claus, the Tooth Fairy, Easter Bunny,) to reward and punish children, just to get them to behave.
The fewer children people have, the less need they feel for church. People who have no children have little need for it at all–after their period of non-church attendance as young adults, they will most likely continue not attending church for the rest of their lives. People with one kid feel some slight need for church, but can survive without it. People with multiple children are eager to send them off to Sunday school for a half hour while they go enjoy the relative peace and quiet of a nice little worship service.
But with birthrates dwindling, smaller households become increasingly atheistic over the generations.
Of course, you may object that there is an obvious causality in the other direction–some religions are explicitly natalist. Mormons, Hasidic Jews, Quiverfull Christians, and Muslims come immediately to mind. However, these groups are a minority among Americans; they can’t explain the overall tendency of religious Americans to have more children. Thus, it seems more likely to me that either the kinds of people who want lots of children also happen to be the kind of people who want to go to church, or that having lots of children actually drives people to go to church.
Stay tuned for Part 3, Religious Identity is Ethnic Identity.