It's not particularly about religion, i.e. going to church and singing hymns.  It's about the tribalism of belonging to a different sect, and it's related to what's gone on in Northern Ireland for years.

If you really think that it's coincidence that supporters of a football club have certain political beliefs, and that this has nothing to do with religion then you need to read up on the history of it all.  The blokes booting a ball around on the pitch are almost irrelevant really.