I think the idea is that in a (small c) conservative community, everybody has the same religion and goes to the same church, say, and that fosters community spirit. In a more (small l) liberal society, people are more free to figure out religion for themselves and do their own thing, but that contributes to social atomisation.

This isn’t specific to religion, that’s just an example (but I think a pretty useful one).

I've heard the related argument that cooperation between people with more differences requires more cognitive effort, so you can get a simple economic or second law of thermodynamics explanation for loneliness. More effort equals less of something.