I think you are missing the whole point of this article.
Let's say you are currently working in a clerical position and you were applying for a leadership position with the Salvation Army and you were in an inteview for the position. The Salvation Army Captain is going through your qualifications for this position and they ask the following question:
"Campion, you've been working with us for a while and you know that the Salvation Army is a Christian Charity organization, so I must ask you this very important question, Campion, are you a Christian?"
Now, think about it.
How would Campion respond?
"Yes, I'm a practicing Catholic."
"Catholic" presupposes "Christian" like "triangle" presupposes "polygon". There are no triangles that aren't polygons, and there are no (observant, faithful, believing) Catholics who aren't Christians.