Francis stood in prayer on the threshold of the basilica’s Holy Door then walked through it, the first of an estimated 10 million faithful who will pass through over the course of the next year in a rite of pilgrimage dating back centuries.

A thin and frail Emeritus Pope Benedict XVI walked through the doorway right after Francis, gingerly negotiating the two steps with the help of a cane and his longtime assistant. It was a rare outing for the 88-year-old Benedict, whose historic resignation led to Francis’ election and a papacy dedicated to showing the merciful side of the church.

Some 5,000 extra police, carabinieri and soldiers have been deployed around Rome, and a no-fly zone imposed on its skies, to protect the pilgrims who are flocking to Rome to participate in the yearlong celebration.

Security was heightened after the Vatican was listed as a possible target following the Paris attacks, but the extra police patrols and traffic stops extended far beyond the immediate vicinity of the Vatican.

Francis launched the 12-month jubilee to emphasize what has become the leitmotif of his papacy: showing the merciful and welcoming side of a Catholic Church more often known for moralizing and casting judgment.

“How much wrong we do to God and his grace when we speak of sins being punished by his judgment before we speak of their being forgiven by his mercy,” Francis told an estimated 50,000 people gathered under rainy skies for his mass opening the jubilee. “We have to put mercy before judgment, and in any event God’s judgment will always be in the light of his mercy.”

The Vatican’s Holy Door, located to the right of the basilica’s main entrance, is decorated with 16 bronze panels depicting the redemption of man’s sin through mercy. Passing through it is meant to symbolize the pilgrimage of life’s journey and the sacrifices endured.

“We have to absorb the message that God always forgives us, which is the message that the pope gives to us,” said Maria Sila, a pilgrim from Buenos Aires as she waited for the mass to begin.

After Francis and Benedict walked through the doorway, the first throngs of pilgrims followed suit, led off by the Italian president.

In a sign that Francis himself was taken aback by Benedict’s frailty when he saw him at the Holy Door, he asked the throngs of pilgrims in the piazza to send their prayers for his “good health.” The crowd cheered.

Holy Years are celebrated every 25-50 years, and over the centuries they have been used to encourage the faithful to make pilgrimages to Rome to obtain an “indulgence” — an ancient tradition related to the forgiveness of sins that roughly amounts to a “get out of Purgatory free” card.

Unlike in Martin Luther’s time, these Holy Year indulgences are free and available to those who pass through the Holy Door.

The last jubilee was in 2000, when St. John Paul II ushered in the church’s third millennium and some 25 million pilgrims flocked to Rome.

The pope made clear from the start that he wanted this Holy Year to be a more sober occasion. For the first time, the pope instructed all cathedrals around the world to open their Holy Doors to pilgrims to encourage the faithful to mark the jubilee at home rather than coming to Rome.

The jubilee year will feature special masses and extra audiences to accommodate pilgrims, while Francis has set aside one Friday each month to perform an act of mercy.