Cardinal Blase J. Cupich said the event was designed as “an opportunity to say that there’s a lot of good that happens in Chicago.” The new pope, the former Robert Prevost, grew up in a modest brick home in a south suburb, Dolton, just on the other side of the city line, and is a confirmed Sox fan.

The couple saw Leo’s election as a good omen for their team, noting that he was photographed days earlier sporting the team’s black-and-white cap with his white papal cassock. With his election, “the White Sox went worldwide,” Ms. Carlson said. “Now they’re higher than the Cubs.” They were both wearing Sox jerseys with “LEO 14” on the back.

The weather was sunny and mild, what might objectively be described as a perfect day. As crowds began to gather outside around noon, the mood was buoyant, with young people playing guitars, drumming and dancing.

Inside, people waited in line to take photographs with cardboard cutouts of Leo in his papal robes, as well as by a fresh mural commemorating Leo’s visit to the ballpark for Game 1 of the World Series in 2005, the last time the White Sox won the title.

The crowd also heard an original song about Leo’s life with lyrics like, “Called into the Vatican dicastery, where he dealt with problems and bureaucracy.”

At the Mass that followed the program, Bible passages were read in Spanish, Polish and English, as well as prayers in Tagalog and other languages. Songs included traditional hymns, a popular Spanish praise song, and the spiritual “Wade in the Water.”

At the culmination of the Mass, nearly 500 lay ministers and ushers dispersed throughout the stadium to distribute Communion.

Cardinal Cupich alluded to the turmoil in his homily. “It is wrong to scapegoat those who are here without documents,” he said, to cheers from many in the stands. “They are here due to a broken immigration system which both parties have failed to fix.”

After the Mass, the music continued, with a break for a familiar ballpark refrain. The crowd laughed with recognition, and then joined in: “Let’s go, Leo,” they chanted. “Let’s go!”