be more Catholic than the Pope (himself)
1. To be more stringently adherent to the laws, tenets, and doctrines of the Catholic faith than is taught or required by the papacy or the Church as a whole. (Usually used hyperbolically as a negative or mocking criticism of someone as being overly pious.) I can't believe we're not allowed to eat meat on Fridays while we're staying with Grandma. She's more Catholic than the Pope himself! Ah, would you stop with your protestations of our mortal sins and telling us to repent every minute of the day! I swear, you're more Catholic than the Pope.
2. By extension, to be more extreme in one's beliefs, behavior, attitudes, etc., than that of the prevailing authority or standard of normalcy. After a year, Jake had started using every dialectic cliché and slang word he had heard in his adopted country, and soon, he was more Catholic than the Pope, so to speak.