In January 2002, prominent Catholics from around the world gathered in Rome to celebrate the Spanish priest who founded one of the church’s most conservative and devout groups, Opus Dei.

The event drew cardinals, bishops and other powerful Vatican officials. And among those invited to speak was a future presidential candidate: Rick Santorum, whose faith had become so essential to his politics that on federal documents he listed the trip, paid for by an Opus Dei foundation, as part of his official duties as a U.S. senator from Pennsylvania.

In a speech at the gathering, Santorum embraced the ideas of Opus Dei founder Josemaria Escriva, who had urged ordinary Catholics to bring an almost priestly devotion to Catholic principles in every realm of life and work.

During Senate debates about abortion, Santorum told the audience in Rome, he hears Escriva telling him that “it is not true that there is opposition between being a good Catholic and serving civil society faithfully.” In his public fight to uphold “absolute truths,” Santorum said, “blessed Josemaria guides my way.”

“ ‘As long as you are making straight for your goal, head and heart intoxicated with God, why worry . . . ?’ ” Santorum said, quoting Escriva, according to a transcript of the speech.

Within the story of how Santorum grew up and decided to run for president, there is the story of a boy who grew up to become ever more devoutly Catholic, a journey all the more relevant as Santorum has vigorously asserted a role for religious conviction in the realm of governance.

On Tuesday, Santorum will face a showdown with Mitt Romney in the Illinois Republican primary, which comes after significant wins in Alabama and Mississippi. In his victory speech last week, Santorum — whose wife has said her husband believes “God is calling” him to seek the presidency — said what he hears most often from voters is “I’m praying for you.”

The man they are praying for was raised in the liberalizing church of the early 1970s and has since taken several turns toward the deeply conservative Catholicism that now anchors his worldview. There was his marriage to Karen Garver and the influence of her devoutly Catholic parents. There was the death of Santorum’s infant son Gabriel in 1996. All have been part of the candidate’s public narrative.

Less well known is Santorum’s embrace of the Catholicism of Opus Dei, a relatively small yet influential group within the church that is defined by the intensity with which followers are urged to live out church doctrine — in Escriva’s words, to “seek holiness” in all realms of life.

The group encourages “unity” between followers’ personal and public lives as Catholics, the rigorous practice of church sacraments and, to some degree, gestures of self-denial. Its most devoted members follow a daily two-hour ritual of wearing a spiked metal chain on their thighs to recall Christ’s suffering — a practice followed by Mother Teresa.

Santorum, whose campaign declined several requests for comment, is not a member of Opus Dei, according to the group, and it is not clear to what degree he adheres to its tenets.

But Opus Dei, whose name is Latin for “Work of God,” has become a significant presence in his life. Santorum has for years attended a church with a number of Opus Dei followers and other affiliations with the group, and he has sent two of his sons to a school run by Opus Dei members. Among his family friends is the Rev. C. John McCloskey, an Opus Dei priest who is a spiritual mentor to many prominent Washington conservatives. McCloskey traveled to Rome with Santorum, led him on a retreat with the group and baptized one of his children.

“He was attracted to Escriva and the spirit of Opus Dei, the idea of lay Catholics . . . giving Jesus Christ a presence in the workplace,” said Monsignor William Stetson, a priest with the organization who knew Santorum after he left the Senate in 2007.

By then, Santorum had come to embrace a version of Catholicism far removed from the one he knew in the early 1970s, when church rituals were relaxing, when Catholic kids were being taught to see moral complexity, and when Santorum, a young teenager then, developed a rapport with a freewheeling Franciscan priest who spoke of Catholicism in terms of moral shades of gray.

Shades of gray

Santorum’s introduction to faith came while he was growing up in the hilly, working-class town of Butler, at a time when parishes were embracing the historic new policies of the Second Vatican Council, which sought to make the church more relevant to a changing world.

Priests who had said the Mass in Latin with their backs turned began facing the congregation and saying it in English. Folk Masses became common. Even God appeared gentler: In one Butler church, a parishioner recalled painters erasing thunderbolts alongside a portrait she understood to be the Almighty and replacing them with a blue sky and angels.

At the veterans hospital where Santorum’s parents worked and his family lived, change came in the form of the Rev. Alex Mullaugh, a priest who was assigned to the chapel on the hospital grounds, where Santorum was the only altar boy. Father Alex, as he was known — a tall figure in sandals and a brown robe — made an impression, said Santorum’s younger brother, Dan.

“He was just a younger, cooler guy,” recalled Dan Santorum. “He’d sometimes come over to the pool. You didn’t think of a priest going swimming, so it was just different.”

A neighbor, Ray Stierer, said Father Alex and his childhood friend “just hit it off.”

“He was always there,” Stierer said, referring to Santorum’s friendship with Mullaugh. “It just came out of nowhere.”

In a recent interview, Mullaugh said he “pushed the envelope” of church tradition, roaming up and down the chapel aisles giving sermons that aimed to “stir people up.”

“I remember saying that there were gray areas,” recalled Mullaugh, who left the priesthood in 1975 and is now a retired computer salesman living in Pittsburgh. “I remember saying that there are a lot more letters in the alphabet than A and Z and we need to use all of them.”

Santorum heard similar lessons at the Catholic school he attended until the eighth grade. In religion classes there and at St. Paul church, where he was confirmed, young priests, some wearing jeans and longer hair, talked about morally complex situations.

“I remember discussions about how it’s not always clear what the right choice is,” said Amy Pierce, a classmate of Santorum’s.

Mullaugh said Santorum absorbed those ideas.

“He just seemed to be so serious,” he said. “Such a serious kid.”

Finding clarity

Santorum’s first turn away from the gray-area Catholicism of his youth came when he met his future wife, Garver, around 1988.

She had only recently ended a six-year relationship with an obstetrician and abortion provider 40 years her senior. Her relationship with Tom Allen — who delivered her in 1960 — had led her to drift from her devoutly Catholic family.

Santorum, a lawyer moving toward his first campaign for Congress, described himself as “a nominal Catholic” at the time.

“I didn’t like the idea of abortion — I knew it was wrong, but I wasn’t sure if it was the government’s business to do anything about it,” he wrote in his 2005 book, “It Takes a Family.”

Then came a dinner at the home of Garver’s parents, and a discussion about abortion, and then, recalled her mother, Betty Lee Garver, “we had them watch a tape that we had, called ‘Meet the Abortion Providers.’ ”

In the video, people introduced as doctors and nurses who ­formerly performed abortions graphically described the procedures, their words accompanied by photographs of what appeared to be dismembered fetuses in trash cans.

“They just sat there crying,” Betty Lee Garver said in a brief interview at her Pittsburgh home. “And they became instantly pro-life. . . . With this and when they started to have children, they started to think more about their faith. I think through the years maybe they watched us,” she said, referring to herself and her husband. “And we are devout Catholics, not cafeteria Catholics.”

Santorum has said that switch to being firmly opposed to abortion had to do with his reading of scientific literature, but also with his religion.

In the Senate, he crusaded against what some opponents call “partial-birth” abortion, saying in a 1996 floor speech that there must be “some sort of moral code in this country.”

At the time, Karen Santorum was pregnant with the couple’s fourth child and began to develop serious complications. Their son Gabriel was born prematurely and died two hours later. They took him home and buried him the next day.

Betty Lee Garver said the loss caused her son-in-law to “go deeper” in his faith. And in the years after, both he and Karen spoke of “God’s purpose” in Gabriel’s short life.

In Karen Santorum’s 1998 book, “Letters to Gabriel,” she includes a kind of exhortation to her husband, writing: “Your daddy needs to proclaim God’s message for life with even more strength and devotion to the cause.”

A ‘turn away from God’

That same year, Rick Santorum met McCloskey, the Opus Dei priest, and began to assert his faith more publicly.

He started a prayer group in the Senate. McCloskey enlisted Santorum’s help in converting then-Sen. Sam Brownback (R-Kan.) to Catholicism. And in 2002, Santorum traveled with McCloskey to Rome for the conference on Escriva.

It was there that Santorum, in response to a reporter’s question, first publicly rejected a speech in which John F. Kennedy, speaking as a presidential candidate in 1960 to voters wary of his Catholicism, affirmed that his religious views would not dictate his public policies. Santorum said it had caused “much harm in America.”

In his address, Santorum embraced Escriva’s view that it is “absurd” to leave one’s Catholicism aside in conducting politics. He said that “as an American, and as a public figure, I am deeply troubled by this turn away from God.”

The crux of the speech was a point that Santorum returned to several times.

“Without a shared belief system that is held and enforced,” he said, “a culture disintegrates into moral chaos.”

For guidance on these matters, Santorum said, he turns to “blessed Josemaria.”

The speech was his first public embrace of the organization Escriva founded in 1928, which now has about 90,000 members worldwide, including 3,000 in the United States.

The group has been criticized in the past by former members as “cult-like” and praised by other members and a succession of popes for its strong commitment to church teachings and loyalty to the Vatican.

About 70 percent of its members are “supernumeraries,” who can marry, while about 20 percent are “numeraries,” who live in celibacy. Numeraries typically wear the spiked chain and perform “the discipline,” occasionally striking themselves with a braided cord as a reminder of Jesus’s suffering.

Brian Finnerty, U.S. spokesman for Opus Dei, said that the group has no expectation with regard to political positions, but encourages “coherence” between religious principles and politics, especially for Catholic politicians.

“Any person who is either voting or acting in public life should base his or her actions on these fundamental principles,” Finnerty said.

In 2002, the scandal over sexual abuse by priests was shaking the Catholic Church, including Santorum’s former parish in Butler, St. Paul, where one of the young, long-haired priests of the 1970s was accused of molesting three boys, including a classmate of Santorum’s, and placed on leave that April.

It is unclear whether Santorum knew that when he gave a speech in July, expressing “profound sympathy” for victims and blaming the abuse on the moral relativism he had just spoken of in Rome.

“Priests, like all of us, are affected by culture,” he said. “When the culture is sick, every element in it becomes infected.”

What was needed, he said, was greater fidelity to church teachings instead of “watered-down versions of our faith.”

‘We are all sinners’

When two of Santorum’s sons were around the age he was when he met Father Alex, he sent them to the Heights, a private liberal arts boys school in Potomac run by Opus Dei members. Its headmaster, Alvaro de Vicente, is a numerary.

When Santorum is home in Virginia, he attends St. Catherine of Siena in Great Falls, one of the few churches in the diocese that host a monthly Opus Dei spiritual meeting. A priest from the group comes in to hear confessions. Santorum often attends the noon Mass in Latin.

“We are all sinners,” the Rev. Alexander R. Drummond said one recent Sunday, faulting Catholics for accepting a world in which “every possible sin [is] exalted.”

In a 2008 speech at Ave Maria University in Florida, Santorum strongly echoed that idea.

He said that Satan has used “the great vices of pride, vanity and sensuality” to corrupt universities, politics and even most Christian churches, except one.

“You say, ‘The Catholic Church?’ No,” Santorum said, explaining that Satan aimed at the country’s Protestant roots. “. . . If you look at mainline Protestantism in this country, it is in shambles. It is gone from the world of Christianity.”

Three years later, Santorum has incorporated those views into a campaign that has steadily gained momentum. On Friday, he traveled to Illinois, the next crucial state in the Republican primary battle. On a day of rallies and fundraising, he spent half an hour meeting with McCloskey, the Opus Dei priest.

McCrummen reported from Butler; Markon reported from Washington. Research editor Alice Crites contributed to this report.