is the pope Catholic
A humorous response to a question that the speaker feels would obviously be answered in the affirmative. A: "Are you going to the party tonight?" B: "Is the pope Catholic? Of course I'll be there!"
Is the Pope (a) Catholic?
used to indicate that something is blatantly obvious. informalis the Pope Catholic?
A positive response (yes) to what one considers a stupid question. Dating from the mid-twentieth century, this answer should be avoided in polite society.