It's common knowledge that Rangers fans are generally Protestants and Celtic fans are usually Catholics.  Apologies if I misunderstood the point you were trying to make, but this is generally what is implied by referring to which team you support...

https://en.wikipedia.org/wiki/Old_Firm

Rangers fans may have historically been more likely to feel aligned with the English, but that was because they were protestants, not because of which football team they supported.

Perhaps it's a comparison to avoid if you don't want to bring religion into it.