There's never been much doubt that Pope John Paul II was destined for sainthood. In more than a quarter-century as the head of the Holy See, he left such an indelible mark that at his funeral in 2005, mourners chanted "Santo subito (sainthood now)."
That road might have seemed less obvious for the other saint-to-be, Pope John XXIII especially for young Catholics who may not be familiar with his relatively short but highly influential papacy, from 1958 to 1963.
"Although few people had as great an impact on the 20th century as Pope John XXIII, he avoided the limelight as much as possible," writes American Catholic.org. "Indeed, one writer has noted that his 'ordinariness' seems one of his most remarkable qualities."
John XXIII, also known as "Good Pope John," was nearly 77 at his coronation and, because of his advanced age, was widely regarded as a "stopgap" pope who wasn't going to make waves. Instead, he called the Vatican II Council, which promulgated one of the most far-reaching and controversial reforms in the Roman Catholic Church's history.
(He also played a peripheral role in the 1962 Cuban missile crisis, offering to mediate between the U.S. and the Soviet Union after receiving a letter from President John F. Kennedy, the country's first and so far only Catholic president.)
Even before the Second Vatican Council, as Vatican II is formally known, he showed a propensity for shaking things up. In one of his first acts as pope, John XXIII moved to dissociate the church from its troubled past relations with Jews. His predecessor, Pope Pius XII, had steered a frequently controversial path through World War II maintaining relations with Nazi Germany while criticizing the war, though historians disagree about the particulars.
Events during the war, especially the Holocaust, led John XXIII to eliminate the description of Jews as "perfidious" in the Good Friday liturgy and to confess the church's history of anti-Semitism.
The following year, in 1959, John XIII called Vatican II, the first ecumenical council to be held since 1870, when what later became known as the First Vatican Council established the doctrine of papal infallibility.
As NPR's Sylvia Poggioli reported from Rome in 2012, Vatican II "opened its windows onto the modern world, updated the liturgy, gave a larger role to lay people, introduced the concept of religious freedom and started a dialogue with other religions." Among other things, it famously decreed that the Mass no longer must be said in Latin but could be given in a language familiar to local congregations.
While the impact of Vatican II might have ensured John XXIII's place in the papal pantheon, a backlash to the sweeping reforms of Vatican II in subsequent years might as easily have jeopardized it.
As Sylvia writes:
"Pope John's successors, Pope John Paul II, who was from Poland, and Pope Benedict XVI, of Germany, were both present at the council and were among those who felt its effects went too far.
"As pope, John Paul, with then-Cardinal Joseph Ratzinger now Benedict by his side, introduced what's known as the restoration and appointed bishops loyal to the Vatican. The major casualty of this restoration was collegiality, the concept that bishops had a role in the decision-making process."
Even so, it was John Paul II who set John XXIII on the path to sainthood by declaring him "Blessed," the next to last step in becoming a saint.
In line with his other reforms, an impromptu address by John XXIII from the window of the Apostolic Palace on Oct. 11, 1962, was informal in tone, unprecedented at the time. It came to be known as the "Speech to the Moon."
"John's words seemed to herald something new: They were spontaneous when popes usually spoke in stiff, prepared paragraphs. They were grandfatherly when popes were supposed to sound regal. And perhaps most importantly, they were beamed into living rooms around the world on the relatively new medium of television."
It is the same sort of spontaneity favored by the current pope, Francis. He has shunned some of the trappings of the papacy, opting to live in a simple residence with other prelates, washing the feet even of lay people and those of other faiths, and often making remarks that seem to take Vatican authorities by surprise. He even poses for "selfies" with fans.
While John XXIII may be less well-known than John Paul II, his canonization stands out in one way in particular: John XXIII will become one of the few people ever to become a one-miracle saint. Francis waived the normal requirement for two documented miracles.
"It is not unprecedented to have a pope waive the second miracle requirement. The last one to do so was John XXIII himself, who in 1960 waived it for St. Gregorio Barbarigo, a 17th-century Venetian cardinal for whom John XXIII had a particular veneration."