A tiny Salvadoran woman, her face shielded by a flowery white veil, walked into Dumbarton United Methodist Church yesterday afternoon, culminating a year of planning by the Georgetown congregation to commit what the Reagan administration considers a crime.

The 200-member congregation is the latest church to take issue openly with federal law and United States foreign policy by offering sanctuary to a refugee from a Central American country whose citizens the present administration says have no reason to flee, other than for economic gain.

"We applaud your great courage," the Rev. Tom Brunkow, the church's pastor, said to the woman as she stood before the congregation. "Now you are here in safety."

The 45-year-old woman arrived in Washington yesterday after a four-day trip from San Salvador, in which she was aided by the sanctuary movement's modern-day "overground railroad." Church members said she flew here from Albuquerque, but other details of her trip were not revealed.

The woman, using the pseudonym America Sosa, said she left El Salvador after her husband was fatally tortured and their teen-aged son arrested, both for what she said was no apparent reason. She will live in the church building at 3133 Dumbarton St. NW for a few days, in a largely symbolic gesture of providing sanctuary, and then move to a more permanent, and secret, location provided by the church.

But the church's action was hardly symbolic. It came only after extensive debate and soul-searching. Some members had to overcome initial doubts, including questions such as why the church's time and money should be directed to refugees instead of the homeless who roam the streets of Georgetown.

Any doubts about their decision may have disappeared at yesterday's joyful and emotional service, which had an ecumenical flavor and included prayers and songs in Spanish by Central Americans and singing led by Peter Yarrow of "Peter, Paul and Mary" fame.

The basic goal of the sanctuary movement is to provide a home in the United States to refugees fleeing El Salvador. Under American immigration law only those refugees who leave their land "with a well-founded fear of persecution" are eligible for political asylum. In the case of many Salvadorans who have entered this country illegally, immigration officials have said that fear is not legitimate and have moved to deport them.

Members of the virtually all-white, middle-class congregation said that it was the compelling testimony of refugees previously given sanctuary by other churches in the Washington area that convinced them to take their action.

Yesterday, Sosa gave testimony of her own, describing her membership in a protest group known as "The Committee of Mothers of the Disappeared."

Speaking through an interpreter, Sosa said that despite reports of a decline in violence by Salvadoran security forces, members of "the mothers' committee" have been under virtual siege in recent months. Sosa said she has been involved with the mothers since her son, then 15, was arrested in 1980, after "walking by a spot where there had been police activity."

She said her son subsequently escaped and is living outside El Salvador, but not in the United States. She said her husband, a bricklayer, died in November 1983, after being arrested and tortured by government police. Her seven children are now scattered through Central America.

Yesterday had further significance for members of the Dumbarton congregation because the sanctuary movement marks March 24, 1982, as its founding.

On that day, the Southside Presbyterian Church in Tucson -- and half a dozen other churches across the country, including Luther Place on Thomas Circle NW in the District -- observed the second anniversary of the assassination of Salvadoran Archbishop Oscar Romero by proclaiming they would harbor refugees from Central American in defiance of American foreign policy.

Since then, almost 200 congregations, representing 12 denominations in 30 states, have declared themselves part of the movement.

How the Dumbarton congregation arrived at its decision to join the movement tells much about the passionate nature of some element's of the church's opposition to American policy in Central America and about the congregation itself.

The decision marked the latest step in the evolution of the oldest Methodist church in Washington from two centuries of comfortable, traditional Protestantism into a hotbed of social activism that belies its chic Georgetown address.

In 1970, the 200th anniversary of the church's founding, Dumbarton's membership had dwindled as families moved to the suburbs. Only a few elderly persons remained, and they were chased off by the arrival of an innovative minister who set the church on its present course. Today, while some of its members are single people who live within walking or biking distance, many come from other parts of the metropolitan area, attracted by its program, rather than its location.

Recently, Dumbarton members have traveled to Central America to study conditions there, produced an antiwar play for children, lobbied against the MX missile, picketed at the South African embassy and supported gay rights.

Brunkow said that by the time he was appointed pastor in 1975, Dumbarton had developed a reputation as "an anti-institutional bunch of renegades," but he insists that all of its positions are based on solid theology.

At the Baltimore Methodist conference last year, he pointed out that his congregation includes retired naval officers, Defense Department scientists, a former adviser to two secretaries of defense; a former NATO division chief and a onetime member of the National Security Council.

"I mention these folks only to balance an impression one might have of Dumbarton Church as a group of naive liberals who can be counted on to support every left-wing cause that comes down the pike," he said last week.

Dumbarton's interest in the sanctuary movement can be traced to Jan. 15, 1984, when the cochairmen of its social concerns committee, Francis Stevens and Nettie Ruth Bratton, heard a Salvadoran couple, identified only as Carlos and Zoila, tell of their harrowing escape after what they said was only their minimal identification with the poor had caused them to be marked for death by forces in their homeland.

"It was a very moving experience," said Stevens, who, after what he said was a career "representing the wrong people" as "an establishment lawyer" in Jackson, Miss., moved here to teach at Antioch Law School.

In the months that followed, Stevens and Bratton, a teacher at Sheridan School in the District, spoke up during a time in the Sunday service set aside for "concerns." Bratton said that, like herself, most of the parishioners were not aware of the situation in Central America. "We'd stand up and say, 'Can you believe this?' " she said.

"It was a long, slow process," Bratton said.

On four Sundays in September, the adult Bible class discussed sanctuary with a professor of ethics at Wesley Seminary at American University; an American Civil Liberties Union lawyer who works with refugees; a member of Langley Friends Meeting, which has sheltered refugees, and Phil Wheaton, an Episcopal priest who directs sanctuary activities in the Washington area.

By Oct. 18, the administrative board called a special congregational meeting to decide.

"We began by listing the pros and cons," said Brunkow, "and at first, the cons outweighed the pros 3 to 1." The prospect of openly defying the government, and then bragging about it, disturbed some members, especially federal employes and laywers. But as the evening progressed, a consensus developed, and when the vote was taken, it was 32 to 3 in favor, with five abstentions.

Still the education continued. After a service on Feb. 24, a lawyer with the Central American Refugee Center (CARECEN) in Adams-Morgan answered questions, including the suggestion that lawyer-members keep a low profile. And yet another refugee, Mauricio, told a chilling story of what he said was his brother's death upon returning to El Salvador, and of his father's disappearance.

CARECEN, founded by Joaquin Dominquez Parada, a Salvadoran lawyer who took refuge in Luther Place upon his arrival here two years ago, estimated that Washington has 65,000 to 85,000 refugees, and is second only to Los Angeles in the number of Salvadoran refugees in the United States.

Verne Jervis, assistant director of public affairs for the Immigration and Naturalization Service, said "there is no targeting of sanctuary groups. They get no special protection either. However, as a matter of policy, we're not going into any churches to make arrests, or even into private homes to look for aliens. We have not come down hard on those people."

In a sermon after the arrests, Brunkow emphasized that the decision to join the sanctuary movement was based on religious, rather than political, beliefs.

"We believe the movement for social justice, in this case providing refuge for victims of war in Central America, is not peripheral to the gospel of Jesus Christ," he said. "The gospel tells of God's loving concern for the disregarded and directs our attention to 'the least' of our brothers and sisters . . . . We believe the Christian must be on the side of those whose lives are put in jeopardy by others, even if that puts one's own life in jeopardy."

Elliott Abrams, assistant secretary of state for human rights and humanitarian affairs, said last week that the moral issue "rests on what happens to the people who are deported. If those people who go back are killed, as the activists maintain, we'd simply change our policy. But that is not true. We have asked the bishop of El Salvador as recently as two weeks ago, and he said it isn't happening. It's an argument not used in El Salvador, not even by opponents of the regime."