Sometimes love is not enough, sometimes you must let the person go anyway.

Tribalism is the main thing people coalesce around (and by extension, against whatever is Not Them). Religion is an expression of that, but not the only one. Nationalism, racism, and xenophobia are in the mix also.

The answer to this according to pop culture and pop music is "why don't we all just get along" but the answer to that question is that we don't like or trust people who are different from us. This is a learned skill. Until people are taught critical thinking and inclusiveness from the cradle, our problems in this regard will persist -- with or without religion.