Is the Pope Catholic?
A jokey rhetorical question.
What we really want to know is 'does the Pope shit in the woods?'
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?"