Is the Pope Catholic?
A jokey rhetorical question.
Like the unquestioned fact that bears shit in the woods, the Pope's Catholicism must be considered beyond doubt - hence the ironic humour in merely asking the question. Apart from the occasional dissident Catholic writer who might ask the question provocatively in order to make a theological point, no one would actually ask the question seriously. Anyone who uses the phrase does so in response to some other query that they wish to answer with a strong affirmative. An example of that, from the earliest citation of 'is the Pope Catholic' that I can find, was printed in The Oshkosh Daily Northwestern in November 1969:
An avid sports fan Tom Pech of Oshkosh was asked before the library board meeting if he planned to attend the Packer-Lion game at Green Bay come Sunday. His reply? "Is the Pope Catholic?"