Catholicism is a sect of Christianity. Stop acting like it isn't. Also, if this figure is correct, I have a lot of roosterslapping to do.
I know it's sect, but of all the Abrahamic religions, those two are the most closely related, thus they are often grouped.
Arguably, Christianity is a sect of Judaism, anyway, if my memory from AP World last year serves me right.
If ye were disagreeing with me, that is.