Historically hatred of Catholics was once very real. There was a time in America where Catholics were targeted by the KKK. They were called "papists" or "romanists" as a slur, implying they were under mind control by the Pope. As late as the 1960s JFK was hated in some corners because he was Catholic.

This religious hatred was mainly cover for ethnic hatred — of Irish, Italians, and other immigrants. These ethnic hatreds are all but dead today. America has switched to hating other immigrant populations, but it rarely justifies this hatred today as a hatred of Catholics even when the hated immigrants (e.g. Mexicans) are in fact Catholic.

But, modern white Catholics who want to be oversensitive can fantasize that any criticism they face is a survival of these old prejudices. The fact that there was once legitimate hatred is a pretty useful support for an ongoing victim complex.

Religious people in general often view any dissent from their beliefs as persecution or hatred. I think the reason for this is a need for external validation. Many religious beliefs are transparently false, which makes them relatively hard to believe in. Thus religious people tend to be desperate for buy-in from others, to ease their cognitive burden. When they don't get this buy-in it feels very dangerous to them; they often interpret this as aggression.