“Who will guard the guards themselves?”
Juvenal (c. A.D. 55-130), The Satires
https://www.poetryintranslation.com/PITBR/Latin/Juvenalhome.php
The question is often used to describe a situation in which a person or body having power to supervise or scrutinize the actions of others is not subject to supervision or scrutiny.
Who is supervising the pope?