Is the Pope a Catholic?
Definition 1 of 1
Pro Tip 1/3Humor Effect
The phrase often carries a humorous tone to emphasize the obviousness of the statement. 
Do I love chocolate? Is the Pope a Catholic?
Pro Tip 2/3Cultural Reference
Understand that the phrase references the well-known fact that the Pope is indeed Catholic, hence its use for rhetorical effect. 
Is she going to wear red lipstick? Is the Pope a Catholic?
Pro Tip 3/3Rhetorical Question
The phrase is used as a rhetorical question, suggesting the answer is so clear that it doesn't need to be answered. 
Will the sun rise tomorrow? Is the Pope a Catholic?