A half-century ago, 40 bishops from around the world gathered in an ancient Roman church and signed a pledge to forsake worldly goods and live like the neediest among their flock.
They were in Rome for the Second Vatican Council in 1965, the deliberations that opened the Catholic Church to the modern world.
The bishops' all but forgotten pledge, known as the Pact of the Catacombs, has gained new resonance with Pope Francis' vision of a church for the poor.
Under the vaulted ceiling of the basilica, a mass is being celebrated to commemorate the pact signed here in 1965. We are just above the Catacombs of Domitilla — many miles of tunnels lined with the tombs of early Christians.
One of the celebrants of the mass is the only surviving bishop of the original 40 who signed the pact, Monsignor Luigi Bettazzi, now 92.
"A group of bishops organized the meeting at the Catacombs of Domitilla ... most of us learned about it by word of mouth," he says.
By signing the Pact of the Catacombs, the bishops pledged "to try to live according to the ordinary manner of our people in all that concerns housing, food, means of transport."
"We renounce forever the appearance and the substance of wealth, especially in clothing ... and symbols made of precious metals," the document said.
Within a few months, some 500 bishops had signed the pact.
But it was soon forgotten, with hardly a mention in the history books about the Second Vatican Council.
One reason, suggested Bettazzi, was that "Pope Paul VI was afraid that too much emphasis on the church of the poor would spill into politics. It was the peak of the Cold War, it could appear the church was leaning toward one side."
Or more specifically, the communist side.
Church historian Alberto Melloni says the pact is probably one of the Catholic Church's best-kept secrets.
"The Pact of the Catacombs is the outcome of long effort at Vatican II to put poverty at the core of the council and this effort failed," he said.
But in one part of the world — Latin America — the pact did not disappear.
Erwin Krautler, the bishop of a Brazilian diocese in the Amazon for 34 years, advocates for the rights of landless peasants and indigenous people. He upholds the principles of the Pact of the Catacombs.
"This pact is an expression of what we call these days, theology of liberation," he said.
Liberation theology is a Catholic grassroots movement that spread throughout Latin America in the 1970s but was scorned by Popes John Paul II and his successor Pope Benedict XVI, who said it was inspired by Marxism. The Vatican disciplined many of its proponents.
Melloni, the church historian, said the Pact of the Catacombs that inspired liberation theology undermined centuries of tradition that had put the Vatican at the center of church power.
Liberation theology "was saying that the center of Catholicism is not Rome, not even the pope, but the real poor, and this was a challenge and the real challenge of this papacy today," Melloni said.
Francis has never specifically mentioned the Pact of the Catacombs.
But his lifestyle — shunning the apostolic palace for a room in a Vatican guesthouse — and his vision of the church as what he calls a "field hospital to heal the wounded," are reviving interest in the 50-year-old document.
Monsignor Bettazzi said he and his fellow bishops planted a seed that is now bearing fruit.
"The Pact of the Catacombs today is ... Pope Francis," he said.