VATICAN CITY — When Pope Francis gave his first full-length interview after his election in 2013, he was asked about the importance of the church providing solid points of reference in a rapidly changing world. The new pope pulled out his thumb-worn breviary and read out a Latin quote from a fifth-century French monk.

Highlighting the words of St. Vincent of Lérins, Pope Francis raised a curtain onto his pontificate: presenting a little-known but once highly influential theologian whose name and citations would soon appear in a number of papal speeches, documents and interviews over the next decade.

The pope’s favorite quote? That Christian doctrine should follow the true and legitimate rule of progress, so doctrine may be “consolidated by years, enlarged by time, refined by age.”

It expresses how doctrine can develop and how there can be growth in the expression and awareness of the faith and in morals “while always remaining faithful to its roots,” he told reporters on the plane to Rome from Morocco in 2019.

This is the point the pope returned to again when speaking to reporters on his flight back to Rome from Canada July 29, when he said St. Vincent offered a “very clear and illuminating” rule for proper doctrinal development.

Like every one one of his predecessors, “Pope Francis has the difficult task of protecting the deposit of faith even while encouraging legitimate growth and progress,” U.S. Monsignor Thomas G. Guarino told Catholic News Service Aug. 3 in an email response to questions.

“For Vincent, the task of the entire church — pope, bishops, theologians, laity — is to foster development and growth over time, but always in full accord with the Gospel and the dogmatic tradition,” said the monsignor, who is professor emeritus of systematic theology at Seton Hall University, South Orange, New Jersey, and the author of “Vincent of Lérins and the Development of Christian Doctrine.”

This idea of growth rooted in and guided by church tradition also led the pope, speaking to reporters on the return flight from Canada, to warn against a perverted sense of tradition, an insidious sin he calls “backwardism.”

People who look to the past do not go forward with the church, the pope said; they lack the root of tradition, which provides life-giving nourishment for growth and development.

Tradition properly understood, he said, is “the root of inspiration for the church to go forward,” not backward. Tradition “is always open, like the roots of the tree, and that is how the tree grows.”

But, just like St. Vincent, the pope recognizes the opposite risk: of going too far and breaking away from the direction of the church as a whole and from church authorities, which he also briefly mentioned on the plane when he upheld his admonishment of the German Synodal Way.

In a letter he sent on the feast of Sts. Peter and Paul 2019, Pope Francis warned against false reforms and walking “alone,” rather than to walk together as “an apostolic body and listen to each other under the guidance of the Holy Spirit, even if we do not think the same way. … The Lord shows us the way of the beatitudes.”

Guarino told CNS that Pope Francis “has put a great emphasis on synodality, that is, on listening to the entire church.”

“He clearly finds inspiration in the thought of St. Vincent who, in his major work — known as the Commonitorium — insists that the Catholic faith is maintained by all Christians” and places great emphasis on the body of bishops throughout the world, “particularly when they are gathered together in a council or synod,” he said.

Pope Francis then seems to rely on this fifth-century monk, not just to guide proper doctrinal development — he also thinks the saint can help church members navigate the world, steering far from the two extremes of an errant, unecclesial drive for change and a dead nostalgia for fruitless tradition.

However, Guarino said, St. Vincent should be not cited, as the pope has done, in matters where there has been “a reversal, such as his teaching in 2017 that the death penalty is ‘per se contrary to the Gospel.’”

The saint described the natural law of progress as analogous to the growth of a body: the child develops into an adult, so “there is change, yes, but there is also stability — with the person remaining the same from youth to old age, even while progressing,” the monsignor said.

Doctrine, too, must follow this “true and legitimate rule of progress,” the saint wrote, so as it matures, nothing new is created and nothing of its essential nature is changed or taken away — just the same way the number of human limbs does not increase or decrease over time.

“It is true that the church’s prior teaching on the death penalty was not a dogmatic statement of an ecumenical council,” and the change can be seen as “an advance in understanding human dignity,” he wrote to CNS.

However, “St. Vincent cannot be used to justify reversals,” the monsignor said. “He is only interested in developments that go forward, clearly maintaining earlier teaching.”

“A good example of the expansion Vincent is talking about is ‘Lumen Gentium’ of Vatican II. The council did not change anything in the traditional teaching about the infallibility of the pope, but added teaching about the infallibility of the college of bishops when teaching together with the pope,” he said. “This was an advance, real progress, which nonetheless maintained prior teaching.”

While encouraging the pope to invoke the saint “with a bit more precision,” Guarino said, “I think Pope Francis should be warmly congratulated for having revived interest in one of the great teachers of the early church.”

The monk of Lérins was clearly committed to protecting the deposit of faith from “profane novelties,” and he “is probably the most thoughtful of all the ancient Christian writers when it comes to growth, development and progress,” Guarino said.