The Rev. Gosbert Rwezahura opened Mass on Sunday morning by saying what everyone in the pews was thinking. “Habemus papam!” he exclaimed at Christ Our Savior Parish in South Holland, Ill. Beaming, he added, “He is one of our own!”

It was the first Sunday in American history with an American pope seated on the throne of St. Peter in Rome. At parishes across the country, Catholics filed into the pews with a sense of wonder, hope and pride over Pope Leo XIV.

At Christ Our Savior, the pride was personal: Today’s parish was formed from others in the area around the South Side of Chicago that includes a now-closed church where the pope attended as a child.

Father Rwezahura put it simply: “We are the home parish of the pope!”

“I’m so full and so proud, I don’t know what to do,” said Janice I. Sims, 75. “I’m definitely blessed because I lived long enough to see it happen.”

Others there traded anecdotes about brushes with the future pope, back when he was known as Robert Prevost: the music director who played at a wedding he officiated, the deacon who went to high school where his mother was the school librarian.

At the standing-room-only 10:30 a.m. Mass at Holy Name Cathedral in Chicago, the Rev. Ton Nguyen began his homily by exclaiming “Viva Papa Leo the 14th!” The congregation applauded. Outside the church, yellow and white bunting hung in celebration.

“My heart is overwhelmed with joy that we have an American Pope, and he is from Chicago,” Father Nguyen said.

Catholics at other services around the country were no less ebullient and were starting to think ahead to their hopes for the new papacy. Perhaps Leo could attract more young people to church, inspire more men to become priests or help unify an often fractious Catholic population in his home country. At 69, he could lead the church for decades.

“He already won over the hearts of the whole world,” said Amelia Coto, 70, who was attending a Spanish-language Mass at Gesù Catholic Church in downtown Miami. “We were without a father, but now God gave us this father we desired so much.”

Ms. Coto is from Honduras, and she teared up when talking about Leo. Like others at Spanish-language Masses in Miami on Sunday, she expressed optimism that a Spanish-speaking pope who lived for decades in South America might be able to sway American immigration policy.

“I hope his arrival will help this new president change, stop all those deportations that Trump is doing to Latinos,” she said.

In New Orleans, the pope’s mother’s family had roots in the Black Creole community, where African, Caribbean and French influences blend. In the city this week, social media feeds were overloaded with images of the pope’s face superimposed in everyday New Orleans scenes. Eating a bowl of gumbo. Showing off his footwork in a second-line parade. Popping his head out of a front door to ask, “How’s your mama and dem?”

Angela Rattler, 69, was attending Mass on Sunday at Corpus Christi-Epiphany Catholic Church in the Seventh Ward. When she first heard the pope speak, tears flowed down her face, she said. “He appears to be such a humble man.”

It was Mother’s Day, which is not a Christian holiday but one where church attendance is usually high anyway. Still, the pews seemed especially full at some parishes.

At St. Ann Parish in Coppell, Texas, all 1,300 seats inside were filled, along with a few hundred people seated in a courtyard at Sunday’s 10 a.m. Mass. The Rev. Edwin Leonard planned a homily that emphasized the vocation of motherhood. But then “the Holy Spirit did a beautiful thing,” he told his congregation, and another topic felt more fitting.

“So it is on Mother’s Day that I’m going to speak about the Holy Father,” Father Leonard said.

Among traditionalists, who had a rocky relationship with the open and informal Pope Francis, some wondered whether Pope Leo might reopen broader access to the traditional Latin Mass. Pope Francis cracked down on the traditional Mass, celebrated by Catholics around the world until the reforms of the Second Vatican Council in the 1960s.

At a Latin Mass at St. Damien Catholic Church in Edmond, Okla., worshipers expressed cautious optimism about the prospect. “There is no way to be sure what he’ll do,” the Rev. Joseph Portzer said in his homily. “But we do see that some of the first words that he said were to talk about unity in the church.”

Father Portzer was among those who found the pope’s American identity intriguing. “We will have an unusual experience being governed by someone who thinks like an American, a Midwestern American,” he said. “It’s going to mean a lot to us to have an American mind-set governing the church.”

For him, that meant a practicality in governing and the possibility that “we will be able, as well, to understand the way he thinks.”

When Father Leonard in Texas heard the new pope’s name on Thursday, the first thing he did was to look up whether he had political or ideological leanings, he told his congregation.

“Mea culpa,” he said in the only Latin words heard during the Mass. “We should not try to fit our pope into our American liberal or conservative camps. If you did that, shame on us.”

Back at Christ Our Savior in the south suburbs of Chicago, a large population of immigrants from Nigeria worshiped along with white and Black families who have lived on the South Side for decades. The pope’s home parish is now a place that in many ways reflects the global church that its favorite son is now charged with leading. Father Rwezahura is from Tanzania, and the deacon serving with him on the altar on Sunday, Mel Stasinski, has lived in Chicago his whole life.

United by a faith shared by 1.4 billion Catholics around the world, they were also connected by their sheer joy on Sunday. As Diane Sheeran, 70, described how she felt when she got the news about Leo: “I had a grin for two days.”

Reporting was contributed by Robert Chiarito in Chicago; Mary Beth Gahan in Coppell, Texas; Breena Kerr in Edmond, Okla.; Katy Reckdahl in New Orleans; and Verónica Zaragovia in Miami.