It’s a trope, like a your Mama joke or “A priest, a rabbi and a __ walk into a bar.” I was at lunch once with a bunch of musicians, and they spontaneously went around the table and everyone told a different Saint Peter joke! That was when I realized, I need at least one solid St Peter joke in my repertoire.

There's a verse in the Bible that mentions something about the gates of hell and has Jesus telling Peter that he's giving him the keys to heaven; some Christians interpret that as meaning Peter is the guardian of heaven and is the one who greets people on arrival and lets them in. A quick google search says it's Matthew 16:18-19.