Question

Go, and return an old Franciscan friar;

That holy shape becomes a devil best. (scene 3, lines 21-22)

What literary term describes these lines?

morality playtragic herocomic relief theme

Answer 1

Short Answer: comic Relief
You have to read around this speech a little bit. Mephistopheles (a demon pretty high up in "the organization") comes in and Faustus makes the comment after all his incantations (which is above your quotation) that he (Meph) is really ugly and is better suited being an old Franciscan Friar -- not a thing you would expect a devil to be.

It's wit and an audience member would smile wryly at the comment. It is not belly shaking laughter. It is just wit, sharp and pointed. (Sort of like the devil himself).

Answer: Comic Relief

One other comment. Try fitting any other answer into those two lines. None of them will work.