The religion the principal characters were taught as a child in working-class Philly often comes up in conversation as they discuss, for example, some of their more shameless schemes. In addition, situations and characters with religious significance arrive on a regular basis. Last Thursday such a character arrived in the form of a priest who had served as the butt of the gang's practical jokes during childhood and adolescence. He provides a conscience figure in response to another of the gang's awful schemes, and then provides a further lesson as one of the group, a young lady on whom he once had a crush, brings disaster on him.
The episode concerns a scheme by the group to make money from donations by Christians after a water stain shaped like the Virgin Mary is discovered in a back room of their bar. Both the scheme and the situation go rapidly downhill from there, and it is all very funny. Yet the wrongness of their quest is never in doubt, and one character's religious qualms about the scheme keeps the story firmly grounded.
In this zany, backhanded way, It's Always Sunny in Philadelphia depicts Christianity in a basically positive way even as it plays mercilessly with its conventions and surfaces.
And as I said, it's definitely funny, as when would-be conman Charlie, pretending to be an evangelist, addresses a small group of pilgrims sitting in the bar:
"Here's a confession: I'm in love with a man. What? I'm in love with a man. A man called God. Does that make me gay? Am I gay for God? You betcha!"