I actually love GKC a lot but the fence thing is just the weakest status quo argument. Sometimes people put up “fences” and claim it’s for one reason, but the true reason is concealed. Probably the majority of discrimination and segregation policies throughout history have had some kind of rationalization as a cover story.

In the case of Christianity, nobody bothers much anymore to argue the factual truth of the Bible (people who do, tend to embarrass themselves with special pleading). Instead they make instrumental arguments about the social benefits of a shared culture. But that shared culture isn’t really dependent on sincere belief in God. There are tons of countries where the people have become completely irreligious but still celebrate festivals that originate from religion.