To old-school Catholics, calling somebody a Lutheran is fightin’ words.
I had a Lutheran great-grandmother. The stories in our family about how the Catholic Church went out of their way to inflict insults on her are legendary.
Sad but true; I’ve got similar stories in my family.
Mom’s side is Catholic; Dad’s side is Lutheran. The family themselves are all very accepting of each other, but the behavior of the Catholic churches! Oi.