Pope Leo XIV on Sunday declared 15-year-old Carlo Acutis a saint, the Catholic Church’s first millennial canonization, during an open-air Mass in St. Peter’s Square that drew tens of thousands of worshippers, many of them young adults and families with children. The ceremony, the first sainthood Mass of Leo’s pontificate, also raised Pier Giorgio Frassati, a well-known Italian layman who died at 24, to sainthood.

The Vatican said 36 cardinals, 270 bishops and hundreds of priests joined the pope for the Mass, underscoring the broad institutional support for the two new saints. Both canonizations had been scheduled earlier in the year but were postponed following the death of Pope Francis in April; Francis had actively advanced Acutis’s cause, arguing the church needed contemporary role models to engage younger Catholics in an era shaped by digital media.

Born May 3, 1991, in London and raised in Milan, Acutis was known for his early aptitude in computer programming and for creating a multilingual website that cataloged Eucharistic miracles recognized by the church. The project, completed while he was still a teenager, earned him the popular nickname “God’s influencer.” He was also known for daily prayer before the Eucharist and a deep devotion that intensified as he grew older.

Accomplished in technology and modest in dress — he is interred in Assisi in casual clothing, often viewed by pilgrims through a glass-sided tomb in jeans and Nike sneakers — Acutis died in October 2006 at age 15 after a brief battle with acute leukemia. Since his death, the Vatican and many lay supporters have promoted his life as a model for young Catholics navigating the opportunities and challenges of the digital age.

"I learned from different people what his professors, his teachers said about his joy and the light he carried around him," said Leopoldo Antimi, 27, who arrived early to secure a spot in the square. "So for me personally as an Italian, even on social networks that are used so much, it is important to have him as an influencer."

The Acutis cause has particular resonance for church leaders who have emphasized Eucharistic adoration. Recent polls cited by the hierarchy suggest many Catholics do not hold a belief in the physical presence of Christ in the Eucharist, and Acutis’s devotion to the sacrament has been highlighted as an example to encourage that practice.

Also canonized was Pier Giorgio Frassati, born in 1901 into a prominent Turin family and known for his works of charity and outreach to the poor. Frassati, who died of polio in 1925 at age 24, has long been venerated for combining a lively social life and friendships with deep commitment to serving others and spreading his faith.

Schools, parishes and pilgrimage sites have already embraced the new saints. In Chicago, an eighth grader named Leo Kowalsky, from a parish named for Carlo Acutis, said the boy’s life offers a relatable model: "It's like I can maybe not be as great as Carlo may be, but I can be looking after him and be like, 'What would Carlo do?'"

The canonizations reinforce a Vatican effort to present saints whose lives address contemporary concerns and who, church officials say, can serve as touchstones for young Catholics navigating faith in an interconnected, technologically driven world. Both Acutis and Frassati were young when they died and have drawn large followings among millennial and younger believers, boosting pilgrimage traffic to sites associated with their lives and graves.

The Mass concluded a public demonstration of support for the new saints across the church’s hierarchy and laity, and it sets the two young Italians as prominent models for a generation the Vatican is seeking to engage more fully in sacramental life and charitable outreach.

Associated press visual reporting contributed to coverage of the event.