It wasn’t just religion it was geography, but United’s success shook that up a bit. All the areas near Maine Rd were City from Moss Side to Didsbury - but a lot of those areas back then had working class Irish communities and support for United was stronger with them, though obviously you had the Gallagher brothers and other Irish Manc Blues, but that made sense because if you grew up in Burnage or Withington or Levy or Longsight they were Blue areas. Similarly Black lads and Asian lads supported City because that was the club associated with Moss Side, Rusholme, Longsight, Fallowfield etc. Sikh lads followed United being from Old Trafford. This had little to do with what was happening on the pitch or even got in the ground, it was what you felt was yours, saw and related to. There was definitely a period when United were seen as not actually that popular in inner south Manchester unless you were Irish, and more of a Salford thing. I was a kid then but that was the way I knew things until United got sexy in the early 90s and we moved out of south Manchester ten years later, so that story has been forgotten by folk but for many the old ties are still there. I always heard we had Masonic ties as well - with sky blue being the colour of heaven!