(I'm neither a theology scholar nor an anthropologist, so I may lack some important background on this.)

I agree that the idea of early church leaders isolating members in order to explicitly limit the introduction of new ideas sounds far-fetched. It strikes me as the kind of thing that would only be said after the fact, by a historian looking for meaning in the details. But attributing those member-isolating rules to something like "preserving group identity" seems like the same thing.

I find myself wondering if something like the anthropic principle is at work here, i.e. the only religious groups to survive that long are the ones who historically isolated their members from outside ideas. There's probably a more general term for what I'm getting at.