While the ceremony itself was remarkable, it was Benedict's presence that added to its historic nature.
Never before have a reigning and retired pope celebrated Mass together in public, much less an event honouring two of their most famous predecessors.
Benedict's presence was also a reflection of the balancing act that Francis envisioned when he decided to canonise John and John Paul together, showing the unity of the Catholic Church by honouring popes beloved to conservatives and progressives alike.
Benedict, 87, arrived in St Peter's Square on his own to cheers and applause from the crowd. Wearing white vestments and white bishops' mitre, he took his seat off to the side with other cardinals but stood to greet Italy's president as he arrived for the Mass.
Italy's interior ministry predicted one million people would watch the Mass from the square, the streets surrounding it and nearby piazzas where giant TV screens were set up to accommodate the crowds eager to follow along. By the time the ceremony began, Via della Conciliazione, the main boulevard leading from the square, nearby streets and the bridges across the Tiber River were packed.
Polish pilgrims carrying the red and white flags of John Paul's homeland were among the first to push into the square well before sunrise.
"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, David Halfar. "It is wonderful to be a part in this and to live all of this."
Pope John XIII, who reigned from 1958-1963, is a hero to liberal Catholics for having convened the Second Vatican Council.
The meetings brought the church into the modern era by allowing Mass to be celebrated in local languages rather than Latin and by encouraging greater dialogue with people of other faiths, particularly Jews.
During his quarter-century papacy from 1978-2005, 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.