Well, I think that there are three things going on-people who were raised Catholic like to think they know everything by osmosis and don’t like the idea that people who have converted know more than they do; some people who were raised Catholic have experienced their Catholicism differently than some people who have converted; ethnicity.
Let me explain what I think about the last two, because its a little bit fuzzy. When I think of what’s important to be about the Catholic Church, one of the first things that springs to mind is the school system. My parents and grandparents all went to and taught at Catholic schools; to me a parish is not right if it doesn’t have a school, and when I imagine the ideal Catholic parish, its located in the heart of a city and plays another Catholic school for the city football championship every Thanksgiving. In other words, I imagine the idealized 1950’s Catholic parish, because that’s the image I learned. Some of us…this part is hard to explain, since “take for granted” is not the right word, since we appreciate the Mass, but idealize that part of it. This isn’t an absolute rule, but that sort of nostalgia is probably less common among converts than among people who were raised Catholic because that idealized urban Northeastern Catholic experience isn’t something they’d have been familiar with growing up. So its a little bit of a gap. And it spills over into other things-a lot of “cradle” Catholics have an instinctive respect for, say, Notre Dame or Boston College or Fordham that converts don’t because those aren’t magic names to them.
The other part is probably…well, bigotry isn’t the right word, but there is probably an instinctive thought by a lot of people in the United States that Smith and Johnson aren’t good Catholic names; Sosabowski and O’Hara and Ippolito are what you should be hearing.
So I think a good deal of it really boils down to nostalgia for the idealized world of Going My Way, and people come off as rude because they don’t realize it is hard to accept that what they experienced as a defining part of “growing up Catholic” is not an essential part of the religion. Just because you didn’t grow up in a neighborhood in Brooklyn and walk to Our Lady of Mount Carmel with all the other Italian boys and listen to the Dodgers on the radio (or listen to your grandfather telling you stories about that time) doesn’t make you a lesser Catholic.