In September 1980, as violence and civil war erupted throughout Central America, a quiet American left Harvard Law School to volunteer with Jesuit missionaries in northern Honduras.

Around him, the United States-backed military dictatorship hunted Marxists and cracked down on the Catholic clergy for preaching empowerment to peasant farmers. But some locals also looked warily on the bearded and mop-haired Midwesterner in their midst.

“Some of the people were wondering what’s going on, who is this guy?” Tim Kaine , then a 22-year-old volunteer and now the Democratic nominee for vice president, said in an interview. He understood why. His mentors in the priesthood had also urged him to be wary of friendly American faces.

Just a few hours south, the Central Intelligence Agency was using Honduras as a staging ground in its covert war against Latin American communism, with right-wing forces training for operations in El Salvador and Nicaragua.

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“It was a time of such intrigue and suspicion,” Mr. Kaine said.

Far from being a C.I.A. operative, Mr. Kaine was a young Catholic at a crossroads, undergoing a spiritual shift as he awakened to the plight of the deeply poor in Honduras. In its far-flung pueblos, banana plantation company towns and dusty cities, Mr. Kaine embraced an interpretation of the gospel, known as liberation theology, that championed social change to improve the lives of the downtrodden.

In Honduras, his recitation of the traditional Catholic mealtime blessing changed to “Lord give bread to those who hunger, and hunger for justice to those who have bread.”

Honduran military leaders, American officials and even Pope John Paul II viewed liberation theology suspiciously, as dangerously injecting Marxist beliefs into religious teaching. But the strong social-justice message of liberation theology helped set Mr. Kaine on a left-veering career path in which he fought as a lawyer against housing discrimination, became a liberal mayor, and rose as a Spanish-speaking governor and senator with an enduring focus on Latin America.

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It also gave Mr. Kaine a new, darker view of his own country’s behavior. “It was a very politicizing experience for me because the U.S. was doing a lot of bad stuff,” he said. “It made me very angry. I mean I still feel it.”

Image Mr. Kaine taught carpentry and welding to 70 vocational students in El Progreso. Credit... via Tim Kaine

Mr. Kaine first went to Honduras during Holy Week of 1974 while a sophomore at Rockhurst High School, a Jesuit academy in Kansas City, Mo. A self-described “sheltered kid” who grew up in a middle class Irish-Catholic family, he delivered donations to a Jesuit mission in the town of El Progreso.

As Mr. Kaine returned to finish high school, race through college in three years and enroll at Harvard Law, the Jesuits back in El Progreso administered to an ever more bloody region.

The Rev. Mauricio Gaborit, a Honduran priest, befriended Mr. Kaine during his initial trip in 1974. Father Gaborit said that after he had helped Nicaraguan refugees streaming over the border, two security officials told him he needed to leave the country if he wanted to live. He did, ultimately coming to study in America.

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“I talked a little bit about this with Tim,” said Father Gaborit, who visited Mr. Kaine at his home outside Kansas City years later and corresponded with him through letters and late-night conversations. Father Gaborit shared his terrifying experiences, but Mr. Kaine, he said, was determined to return.

He “wasn’t a worried person,” Father Gaborit said.

Instead, Mr. Kaine was worried that he was rushing through life. While at Harvard Law, he arranged to volunteer again in El Progreso. His parents wondered if he was considering the priesthood, but he said he was seeking something else.

El Progreso seemed as far from Kansas City as it was from Rome. Priests wore short sleeves and had muddy boots. Bishops distinguished themselves with white guayabera shirts.

Upon Mr. Kaine’s return in 1980, the Jesuits told him his experience working in his father’s metal shop was more valuable than his legal knowledge. He was soon teaching carpentry and welding to 70 vocational students.

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The Rev. Jack Warner, a priest with the mission, recalled Mr. Kaine as quiet and methodical, which came in handy for his record-keeping duties at the school. But Father Warner also described an atmosphere of fear in which the most basic advice was “be careful of who you are talking to — most particularly the Americans.”

Father Warner himself saw the church’s role as advocating for the poor and the oppressed — as he saw it, “the gospel is an extremely communist document.”

Image Mr. Kaine and his wife, Anne Holton, leaving St. Elizabeth’s Church, a predominantly African-American congregation in Richmond, Va. Credit... P. Kevin Morley/Richmond Times-Dispatch, via Associated Press

It was just that blending of Catholicism and socialism that raised suspicion in the Reagan administration and the Honduran military. At the Vatican, Pope John Paul II, whose defining experience was resistance to the communist regime in Poland, frowned on liberation theology.

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“That’s one of the reasons we didn’t make too much noise about it,” Father Warner said.

But Mr. Kaine, who said the priests felt “like they were under the church’s thumb,” would get exposure to — and seek out — some of the leading thinkers of the movement.

He met Jon Sobrino, a Spanish theologian, an adviser to the murdered archbishop of El Salvador, Oscar Romero, and the author of “Jesus the Liberator,” which was criticized in 2006 by the Vatican’s Congregation for the Doctrine of the Faith on the grounds that it might “cause harm to the faithful.”

During a short stay in Nicaragua, Mr. Kaine looked up an American, the Rev. James Carney, known there as Guadalupe, who had been exiled from Honduras in 1979, in part for adopting an extreme view of liberation theology that supported the taking up of arms against military oppressors.

Mr. Kaine hopped off a bus in northern Nicaragua, walked miles to Father Carney’s remote parish and spent a memorable evening listening to the priest describe “both getting pushed around by the military and getting pushed around by the church.”

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The stigma of “communist priests” hung over all the Jesuits in the region, and some priests and nuns were killed. But most of the American Jesuits Mr. Kaine worked with on a daily basis had a more pragmatic streak and rolled their eyes during philosophical debates about liberation theology.

Mr. Kaine became close to the Rev. Jarrel Wade, known as Father Patricio, a large, athletic American who cheerily said the dirt floors he slept on in Honduras reminded him of camping trips.

During the rainy Christmas season, Mr. Kaine accompanied Father Patricio as ecstatic children in rural towns greeted their mules in anticipation of getting gifts of peppermints. In one of those remote villages, a poor family with malnourished children gave Father Patricio a Christmas gift of food.

Mr. Kaine did not approve, and when they were alone, Father Patricio explained, “‘Tim, you have to really be humble to take this gift of food from a family that poor.’” The message, as Mr. Kaine understood it, was that if you consider someone incapable of giving, you strip them of their humanity.

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“I think about it all the time,” Mr. Kaine said.

On the way back, their Jeep repeatedly got stuck in the mud. They ended up alone in a one-room schoolhouse with a can of peanuts, a bottle of wine and Kenny Burrell’s “The Little Drummer Boy” playing on their transistor radio.

“‘Tim, no offense,’” Mr. Kaine recalled Father Patricio as saying. “‘This is the worst Christmas I’ve ever had.’”

But Father Patricio, who died in 2014, was aware that the lessons he was imparting to his young companion might pay off. “Patricio was impressed,” said his brother, the Rev. John J. Wade, “that this guy might go back to the East Coast and have influence.”

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The nine months spent volunteering in Honduras made a formative mark on Mr. Kaine and inspired him to write a poem, “Still Life,” in a 1981 newsletter connected to the mission.

In the saddest slum of San Pedro

lives are played out in the shade of a highway

where buses glide like lost thoughts overhead

From the clutter of roofs sprout antennae

fed by a seethe of thick misery within.

This spider’s quilt of questionmarks;

each crooked finger

tests the wind,

each predicts change

that just won’t come.

In 1983, the year Mr. Kaine graduated from Harvard Law, Father Carney, acting as a chaplain to a group of rebel fighters, sneaked back into Honduras, where he died under murky circumstances, intensifying Mr. Kaine’s doubts about the United States’ role in the region.

In his letters to Father Gaborit, the priest who had fled Honduras, he expressed anguish at a foreign policy that both promoted liberty and supported covert activities that undermined American values.

“He questioned very much,” Father Gaborit said.

Upon settling in Richmond, Va., with his wife, Anne Holton, and joining St. Elizabeth’s, a predominantly African-American Catholic church, Mr. Kaine told his pastor that his exposure to liberation theology had “changed him, it deepened him.” The pastor, Monsignor Michael Schmied, said Mr. Kaine inspired him to leave for El Salvador in 1987.

Mr. Kaine has remained supportive of the El Progreso mission and recently visited the graves of his mentors there. He now sees a parallel between them and Pope Francis, the first Latin American pope, a Jesuit who was fluent in liberation theology, faced an oppressive government in Argentina and wrestled with the hierarchy in Rome.

“I really feel I know him,” Mr. Kaine said. “The age he was in 1980 and ’81 was about the same age as a lot of my friends were. The Jesuits.”

Correction: Sept. 2, 2016 An earlier version of a capsule summary for this article misstated the region where Honduras is located. It is Central America, not South America. Sept. 2, 2016

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