The Mass was inherently the same as any other Sunday service - with just a lot more people filling the pews.
"I'll tell you why all these people are here, because you were invited," the Rev. Philip Bochanski, 40, said during the homily. "Whether there are as many people next week as there are this week is entirely up to you."
Mass mobs - a trend that started last year in Buffalo, N.Y. - have tried to worship at older city churches that struggle to get parishioners due to changing neighborhoods. St. Francis Xavier was surrounded by a majority Irish Catholic population during its construction between 1894 and 1898.
"Just this neighborhood in particular, back in the 1950s, '60s it was maybe 90 percent Catholic," the Rev. Paul Convery, 60, said. He has been there since 1990.
Fifty-three percent of Philadelphians were affiliated with a religion in 2000, 61 percent of whom identified as Catholic, according to the Glenway Research Center. It also found 21 percent of Pennsylvania's Catholics live in Philadelphia.
Of the 16,221 people living within St. Francis Xavier's parish boundary, 13 percent were registered Catholic in 2010, according to the archdiocese. The population was roughly 72 percent white, 21 percent black, 5 percent Hispanic and 4 percent Asian that year, the archdiocese said.
Fairmount as a whole had declines in black and Hispanic populations between 1990 and 2010. Fairmount North's white population quadrupled in that time, while the Asian population of Fairmount South more than tripled, according to the Philadelphia Research Initiative at the Pew Charitable Trusts.
Sunday's turnout was noticeably younger than usual. Ann Post, 47, went with her husband and four children.
"There was a family in front of us and there must have been 10 kids under the age of 8," said Post, of Oreland.
It became clear who the newcomers were when many remained seated for Holy Communion, waiting for an usher to tell them when to stand - while regular parishioners proceeded up the center aisle.
"You figure it out - what's the pattern at this church," Post said.
The Scanlon family of Pickney, Mich., enjoyed the ornateness of St. Francis Xavier, with its carved marble columns, colorful stained glass windows, and painted ceiling.
"The church we go to is old, but it's much smaller," said Christie Scanlon, 30. She and her husband, Justin, brought their three young children to town to visit Justin's parents. "We want the kids to appreciate different styles and different types of worship," she said. Justin Scanlon worshipped at the Victorian-style church while attending Drexel University.
Ian Scanlon, 2, enjoyed counting the congregants in the church. "He kept saying, 'There's so many friends here,' " said his mother, a doctoral student at the University of Michigan.
"At this time in this day when Catholicism is getting pushed against the wall, it's good to see people come out and say it's OK to be Catholic," said Suzanne Rogers, 43, an operating-room nurse from Westmont, N.J., and a former parishioner of St. Francis Xavier. "It was refreshing," she said of Sunday's turnout.
Archbishop Charles J. Chaput, Mayor Nutter, and Gov. Corbett left Philadelphia Sunday for Rome on a mission to persuade Pope Francis to attend the World Meeting of Families in September 2015 in Philadelphia.
There's little doubt the social media savvy pontiff would have enjoyed Sunday's Mass mob in Fairmount South. He called the Internet "a gift from God" in a statement released in January. His selfies taken with followers at the Vatican exploded across Twitter last year.
Correction: An earlier version of this story incorrectly described the crowd count at Sunday's mass.