St. Peter's Square was packed with faithful on Sunday as Pope Leo XIV presided over the canonisation of Carlo Acutis, a London-born teenager widely described as a millennial figure in the Catholic Church, and Italian layman Pier Giorgio Frassati.

Scores of pilgrims had been sleeping in tents and lined the streets around the Vatican, many carrying banners, posters and images of Acutis, as they waited to enter the square for the long-anticipated ceremony. Dozens travelled from Assisi on a special train to attend the Mass and to join others watching on large screens in the medieval Umbrian city, where Acutis's remains rest.

Carlo Acutis, who died of acute leukemia in 2006 at age 15, was raised to sainthood after the Vatican recognised two miracles attributed to his intercession. Born May 3, 1991, to an Italian mother and a father of half-English, half-Italian descent, Acutis grew up in Milan. He earned attention within the church and beyond as a self-taught coder who used his skills to document reported miracles and stories about saints online, and was often portrayed in media accounts as a young person who combined devotion with an aptitude for technology.

His mother, Antonia Salzano, has said he had a "special relationship" with God from an early age. In writings attributed to him, a seven-year-old Acutis described a life plan "to be always close to Jesus." He attended daily Mass, helped bullied children, and assisted homeless people by bringing food and sleeping bags, according to accounts from his parish in Milan. His final wish, the family said, was to be buried in Assisi, the town associated with St. Francis.

Acutis's tomb in Assisi is enclosed in glass; visitors often find it notable that he is interred wearing jeans and a pair of Nike trainers. Pilgrims and tourists visit the shrine in large numbers, and souvenirs and mementos bearing his image have been sold in the town.

The path to sainthood for Acutis followed the church's formal process, which requires documented miracles attributed to the candidate's intercession after death. The Vatican accepted the first of two miracles in 2020, the healing of a Brazilian child, Mattheus Vianna, who had suffered from a rare pancreatic malformation. Vianna's family says he visited Acutis's tomb in Assisi in 2012, touched a relic and was cured; medical experts later documented the recovery.

A second miracle accepted by the Vatican involved a Costa Rican student, Valeria Valverde, who sustained a critical head injury in a cycling accident in Florence in 2022. After relatives prayed at Acutis's tomb in Assisi, Valverde began to breathe independently and ultimately made a recovery that church investigators concluded was inexplicable by medical science, paving the way for the canonisation.

Acutis was declared "venerable" in 2018 and was beatified in 2020. His canonisation had been scheduled earlier in the year but was postponed; the Vatican rescheduled the ceremony for Sunday.

Also canonised on Sunday was Pier Giorgio Frassati, an engineering student and mountaineer who died of polio in 1925 at age 24. Frassati, known for his commitment to serving the poor and sick of his city, was beatified by Pope John Paul II in 1990. The Vatican said it recognised a second miracle needed for his canonisation in 2024, involving the unexplained healing of a young American man who had been in a coma.

The decision to canonise both men reflects a broader effort within the church to highlight models of holiness who combined ordinary lives with devoted service. Frassati has long been honoured as a lay example of charity and enthusiasm for the outdoors; Acutis has been portrayed as a bridge between contemporary youth culture and traditional Catholic devotion.

Cardinals and clergy at the Mass emphasised themes of service, prayer and witness. Pilgrims at the Vatican held images of Acutis and Frassati and prayed as the pope delivered the homily and performed the rites that declare a person a saint. Supporters of Acutis have used nicknames such as "God's Influencer" to describe his online work documenting Eucharistic miracles and other faith traditions, language that has circulated widely on social media and among younger Catholics.

Medical organisations note that leukemia, the blood cancer that killed Acutis, affects bone marrow and blood cells and typically causes symptoms such as fatigue, bruising, infections and fever. Treatments for various forms of leukemia have improved over recent decades, but outcomes vary by type and patient.

The canonisations are expected to reverberate in both Rome and Assisi. In Assisi, authorities and local churches organised public viewings of the Mass and welcomed visitors to Acutis's shrine, continuing a flow of pilgrims that has grown since his beatification. In Rome, the ceremonies brought thousands to St. Peter's Square and to nearby churches and piazzas, where local vendors sold images and memorabilia related to the new saints.

The Vatican's declaration follows a multiyear investigation into both men's lives and the alleged miracles attributed to them. Final approval of canonisations rests with the pope, after review by Vatican congregations and independent medical and theological experts who assess whether purported healings lack natural explanation.

After the Mass, pilgrims planned to return to Assisi and other hometowns to mark the occasion. For many who travelled to Rome, the ceremony offered a chance to celebrate religious devotion expressed through modern forms of witness as well as through longstanding traditions of charity and sacrifice.