Pope Francis has declared his two predecessors John XXIII and John Paul II saints in an unprecedented canonisation ceremony made even more historic by the presence of retired Pope Benedict XVI.
Pope Francis recited the saint-making formula in Latin, saying that after deliberating, consulting and praying for divine assistance "we declare and define Blessed John XXIII and John Paul II be saints and we enrol them among the saints, decreeing that they are to be venerated as such by the whole church".
Pope Benedict was sitting off to the side with other cardinals in St Peter's Square during the rite at the start of Sunday's Mass. He and Pope Francis briefly greeted one another after Pope Francis arrived.
Never before have a reigning and retired pope celebrated Mass together in public, much less at an event honouring two of their most famous predecessors.
Pope Benedict's presence was a reflection of the balancing act that Pope Francis envisioned when he decided to canonise Popes John and John Paul together, showing the unity of the Catholic Church by honouring popes beloved to conservatives and progressives alike.
Pope Francis made that point clear in his homily, praising both men for their work associated with the Second Vatican Council, the ground-breaking meetings that brought the 2,000-year-old institution into modern times. Pope John convened the council while Pope John Paul helped ensure its more conservative implementation and interpretation.
"John XXIII and John Paul II cooperated with the Holy Spirit in renewing and updating the church in keeping with her pristine features, those features which the saints have given her throughout the centuries," Pope Francis said.
He praised Pope John for having allowed himself to be led by God to call the council, and he hailed Pope John Paul's focus on the family - an issue Pope Francis has taken up himself.
"They were priests, bishops and popes of the 20th century," Pope Francis said. "They lived through the tragic events of that century, but they were not overwhelmed by them."
It was Pope Benedict who put Pope John Paul on the fast-track for possible sainthood just weeks after he died in 2005, responding to the chants of "Santo Subito!" or "Sainthood Now!" that erupted during his funeral Mass. His canonisation is now the fastest in modern times.
Pope Francis then tweaked the Vatican's own saint-making rules, deciding that Pope John could be made a saint without the necessary second miracle usually required for canonisation.
Pope Francis took a deep breath and paused for a moment before reciting the saint-making formula in Latin at the start of the ceremony, as if moved by the history he was about to make in canonising two popes at once.
Applause broke out from a crowd that stretched from St Peter's to the Tiber River and beyond.
"This is such a historic moment," marvelled the Rev Victor Perez, who brought a group from the John Paul High School in Houston, Texas and waited for nearly 12 hours to get near St Peter's. "John Paul was so impactful on the church. He completed the work of Vatican II. Today honours the last 50 years of what God has done in the church."
In Pope John Paul's native Poland, bells tolled as soon as Pope Francis pronounced the two men saints.
"He changed Poland and he changed us with his teaching and with his visits here," an emotional Maria Jurek said as she watched the proceedings on giant TV screens at a sanctuary dedicated to John Paul in Krakow.
Yet the atmosphere in St Peter's seemed sombre and subdued - perhaps due to the chilly grey skies and cumulative lack of sleep - unlike the party atmosphere of Pope John Paul's May 2011 beatification when bands of young people sang and danced in the hours before and after the Mass.
The Vatican estimated that 800,000 people watched the Mass in Rome, with about 500,000 in the square and nearby streets and the rest watching on TV screens that had been set up in piazzas around the city.
By the time the ceremony began, Via della Conciliazione, the main boulevard leading from the square, nearby streets and the bridges across the Tiber were packed.
Polish pilgrims carrying the red and white flags of Pope John Paul's beloved homeland had been among the first to push into the square well before sunrise, as the human chains of neon-vested civil protection workers trying to maintain order finally gave up and let them in.
"Four popes in one ceremony is a fantastic thing to see and to be at, because it is history being written in our sight," marvelled one of the visiting Poles, Dawid Halfar.
Pope Benedict had promised to remain "hidden from the world" after resigning last year, but Pope Francis has coaxed him out of retirement and urged him to take part in the public life of the church.
During the Mass, Pope Benedict sat off to the side of the altar with other cardinals, though he was clearly in a place of honour. He received the Italian president and a steady stream of cardinals, as well as Pope Francis himself at the beginning and end of the service. Pope Benedict had arrived in the square on his own to cheers and applause, wearing white vestments and white bishops' mitre.
In a dress rehearsal of sorts, Pope Benedict attended the February ceremony in which Pope Francis installed 19 new cardinals. But celebrating Mass together with Pope Francis was something else entirely, a first for the institution and a reflection of Pope Francis' desire to show the continuity in the papacy, despite different personalities, priorities and politics.
Pope John XIII, who reigned from 1958-1963, is a hero to liberal Catholics for having convened Vatican II, which allowing Mass to be celebrated in local languages rather than Latin and encouraged greater dialogue with people of other faiths, particularly Jews.
During his quarter-century papacy from 1978-2005, Pope John Paul II helped topple communism through his support of Poland's Solidarity movement. His globe-trotting papacy and launch of the wildly popular World Youth Days invigorated a new generation of Catholics, while his defence of core church teaching heartened conservatives after the turbulent 1960s.