There are some big gigs in stand-up comedy — headliner at Madison Square Garden; host of the White House Correspondents’ Association dinner — and then there’s the show that Jim Gaffigan is about to play. On Saturday, Mr. Gaffigan will be the lone comedian on the bill at the Festival of Families, a Philadelphia event that will be visited by Pope Francis. With Mark Wahlberg as host, the festival will also feature Aretha Franklin, Andrea Bocelli, Juanes, Jackie Evancho and Sister Sledge; it is expected to draw more than one million attendees.

For Mr. Gaffigan, 49, the co-creator and star of “The Jim Gaffigan Show” on TV Land and best-selling author of “Dad Is Fat” and “Food: A Love Story,” this performance is a source of considerable pride and maybe just a little anxiety. On Monday, he spoke about how he is preparing for the festival and how his faith is distinct from his humor. These are edited excerpts from that conversation.

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