I and my classmates in parochial high school in the 1960s probably did much the same kind of stuff. The difference now is that you can take photos and post them for everyone to see. If that had been the case back that we probably would have done the same.
He probably wasn’t doing it to show disrespect, but to get a laugh from his friends. He posted it on Facebook to show it to his friends, not to shock the devout. Odds are he probably didn’t even take the latter into consideration.
Kids that age do stupid things. If you don’t hire anyone who did stupid shit when they were a teenager you’ll have a very small pool to hire from.