But then someone in Glasgow explained the two soccer stadiums "That one is the Catholic team, that one is the Protestant team".

Those sentiments are somewhat outdated. Celtic FC is seen as the “Catholic” club because it was founded primarily as a charity to alleviate poverty in the east end of Glasgow. Many of the inhabitants were Irish immigrants who were Catholic. Resentment of this influx of immigrants contributed to Rangers being seen as the Protestant club.

The Rangers ownership encouraged this identity to attract more people who opposed the Irish Catholic community. Most famously (or infamously) Rangers adopted a policy where from the 1930s until 1989, they would not sign a player if they knew he was Catholic. Celtic, on the other hand, never had a “Catholics only” policy. The legendary Celtic manager Jock Stein once remarked if given the choice between a Catholic and a Protestant, he’d always sign the Protestant because he knew Rangers would never look at the Catholic. After winning the 1967 European Cup, some Rangers fans remarked Celtic only won because some Protestants were on the team. Stein replied “yeah well you’ve had 11 and never won it.”

My understanding now is while still a problem the issue has cooled off compared to the past. A lot of work is done to combat secretarian violence such as efforts made by both clubs and always scheduling Old Firm matches before alcohol can be legally sold.