"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.
See #43.
It might presuppose anything you want, but it ignores the tradition of the person with whom this man was conversing.
Why couldn't you just say "yes" and leave it at that?