The Diocese took issue with the unnamed student's kinky twist on the familiar costume, which involved being half-naked from the waist down and shaving her pubic hair in the shape of a cross.
She was also reportedly handing out condom to attendees of the school's 4th Annual Anti-Gravity Downhill Derby.
"I think obviously all of us know that when we're growing up we do stupid things," Bishop David Zubik told local CBS affiliate KDKA, "but to cross over the line in this instance shouldn't happen with anybody."
Zubik says he waited until now, two weeks after the derby, to publicly protest, because he wanted to talk it over with the school first.
"What I do want to have happen is for this person to learn an important lesson," he said.
It's unclear, however, if the school intends to take any action.
In a statement to the press, a CMU rep said the university was reviewing the incident and would take "appropriate action" if any of its community standards or laws were violated.
CMU students, meanwhile, think this is a freedom of speech issue and the school has no right to punish the coed for it.
"It's all in good fun and it's not meant to harm anyone," student Ivy Kristov told KDKA.
[photo h/t: Barstool U]