Finny.....I agree......"I'm a good Catholic girl" seems to be a cliche, a joke, a touch of doublentente.....a snappy answer.
But as you quite rightly point out, other religions don't use that.
I suspect that sqad will have known a few 'good catholic girls' in his time :-)
One of the worst things about living in the West of Scotland is some folk judge you on which football team you support and then leap to the wrong conclusion about one's religion.
Finny - are you a Prod or what LOL - can recall this big time. I was 13 and had never met a Prod - however one of my aunts took me into unfamiliar land and I remember saying to her "Oh Doreen I am afraid am afraid of the Prods. - I am sure Doreen *** herself.
My first encounter with a Prod when I was 16 and started work.