No one said that the religious belief itself is the important factor, and your first guess of it enforcing terrible marriage is insultingly reductive. Your second guess only comes close. It's literally the church, the meeting place where you see the same people every week. Kids meet other kids, adults meet adults. If you think that every person in the building actually believes what the pastor says then i think you have not been to most American churches. The shared belief is a part of it but can be easily faked if necessary. The entirety of social life from the church is the proximity. The vast majority of friends made from that proximity do not spend even half their time together speaking about the church or the belief, especially the children.

Sure, the church has problems, as does religion. All that's being said is that church is the last great, high trust, free, "third space" in America. The fact that it houses a religion is related but not central to that fact.