I'm basically a secular humanist, and I've heard the statement that Catholics aren't Christians, in person, a few times. The two times that come to mind were from very different people (a Chinese Christian that lives in Beijing, and a Canadian Christian that lives on an apple orchard in southern Ontario). Both of whom were coworkers I spent some time with while travelling for work (different jobs, about 10 years apart).
I've always shut it down as a wildly offensive thing to say, and not worthy of discussing. So I've never gotten a real explantion for why some Christians believe it. Is it a common opinion?
Just to spell it out for people: myriad means 10,000 but has been used as a stand in for "huge variety" for so long that people don't know that anymore.