From Longman Dictionary of Contemporary EnglishIs the Pope (a) Catholic?Is the Pope (a) Catholic?informalCERTAINLY/DEFINITELY used to say that something is clearly true or certain – used humorously ‘Do you think they’ll win?’ ‘Is the Pope Catholic?’ → Pope
Examples from the Corpus
Is the Pope (a) Catholic?• Is it a flop? Is the Pope a Catholic?• Sure? Is the Pope a Catholic?• The answer to that question would seem to be another question, namely and to wit: Is the pope a Catholic? 