The Reagan administration is on a collision course with the mainstream of Christianity in the United States, Roman Catholic and Protestant alike, over its policies in Central America.

The mainline churches, from local congregations to top officials, in demonstrations, resolutions and in some instances violating the law, have shown more determination in their opposition to the nation's foreign policy, particularly in regard to El Salvador and Nicaragua, than at any time since the Vietnam war.

Although the fine points vary from church to church, the religious leaders agree on key demands: U.S. support for a political as opposed to a military solution in El Salvador; an end to efforts to destabilize the government of Nicaragua; the involvement of other nations--Mexico, Colombia, Brazil, Venezuela, and Spain--in resolving Central American conflicts instead of the United States acting as sole power broker, and changes in U.S. immigration policy to provide temporary asylum for refugees from violence and repression in Central America.

The administration has said a hard line is necessary to contain leftist aggression and the threat of communism all around the world, and administration officials have made the point to church groups that communist regimes are by definition anti-religious. Nevertheless, nearly every convention of a mainline church this spring has adopted some kind of resolution critical of the administration's role in Central America.

Local churches in more than 60 U.S. cities are defying immigration law by sheltering illegal immigrants, mostly Salvadorans who fled the bloodbath of their homeland. The sanctuary movement--an invoking of the ancient tradition that the fugitives are immune from arrest in the house of God--began as a response to the plight of refugees in southwest border cities. But it also "has a political dimension," said the Rev. Dr. John Steinbruck of Luther Place Memorial Church in Washington, "of trying to save the nation from shooting itself once more in the kneecaps."

Delegations of church leaders have been pressing the State Department and members of Congress for policy changes. At the grass-roots level, church leaders lobby legislators.

Committees in hundreds of communities are staging vigils, circulating petitions, running speakers' bureaus and organizing tours of Central America.

A North Carolina-based group has recruited some 150 church members from a variety of denominations from 30 states and flown them to Nicaragua. The group originally planned to spend the Independence Day weekend dispersed along the Honduran-Nicaraguan border as a "human shield," but Nicaraguan officials were "not real comfortable about the idea, security-wise," said Peggy Olney of Raleigh, one of the organizers. Instead, the group will hold a series of peace vigils in more secure sites.

Unlike the era of the Vietnam war protests, Roman Catholics, and particularly the U.S. bishops, are in the vanguard of opposition to this nation's policy. They have adopted position papers, and high-visibility prelates such as Archbishop James A. Hickey of Washington have testified before congressional committees.

The murders of Salvadoran Archbishop Oscar Romero and four American churchwomen, also in El Salvador, in 1980 have been remembered by many Protestants and Catholics in this country in prayer vigils and protests. Last December, 11 nuns were arrested in Washington during a pray-in to protest continued U.S. military aid to El Salvador in light of its record on human rights.

Protestants and Roman Catholics in this country receive information from Central America through missionaries there, many of whom serve the poor, and these missionaries' views often differ with those of government officials.

This spring, an interdenominational group numbering about 50 Protestant and Catholic missionaries in Nicaragua issued a stinging response to President Reagan's April 27 address to a joint session of Congress in which he warned of a impending crisis in that country.

"We who live here can attest to the fact that the crisis already exists and daily is made more acute by U.S. policy in the region," the missionaries said, charging that Reagan "presented a very distorted view of the Central American reality to the American people."

Condemning "U.S.-backed counterrevolutionary activity" in Nicaragua as "an illegal and undeclared war by the Reagan administration," the missionaries said: "We have seen tobacco fields burned, young volunteers in the coffee harvest shot and killed . . . . We have taken shrapnel out of the heads of babies, from mortar shells launched from Honduran territory."

Unlike Vietnam, Central America is close enough for North Americans to visit and observe conditions for themselves. "There are an incredible number of groups going to Central America, sometimes organized by churches, sometimes on their own," said the Rev. Oscar Bolioli, a Bolivian Methodist on the Latin American desk of the National Council of Churches in New York. "Most of them come back with strong support for changes in Latin America," he said. "They can see that the plans of the United States are wrong, that they don't work."

Some of the visitors are official fact-finders, such as the Presbyterians' Central America task force, which last November touched base with a wide range of people--missionaries, U.S. diplomatic officials, governmental leaders in El Salvador, Nicaragua, Honduras and Guatemala, church people "on both sides" and political prisoners.

Last month, the Presbyterians endorsed the task force's lengthy report, which appealed to the U.S. government to press for negotiated rather than military solutions in El Salvador, to support self-determination in Nicaragua and resume economic aid to that country, and to withdraw military advisers from Central America.

Sometimes, personal visits to Latin America produce a personal effect. Two years ago, the Rev. Richard Lundy of St. Luke's Presbyterian Church in the affluent Minneapolis suburb of Wayzata toured Latin America, and the next year 18 members of his congregation went to Mexico for a 10-day session on Latin America.

Last December, the 400-member congregation offered sanctuary to a young Salvadoran who deserted his country's army because "he came to the conviction that what he was doing was immoral," Lundy said. Now, Lundy reports, the Salvadoran is giving the congregation a new understanding of the situation in his homeland.

The churches do not agree on the issue of sanctuary, which despite tradition is a violation of the law. "Some Catholic bishops have endorsed it," said Tom Quigley, Latin America expert for the United States Catholic bishops conference. "Others prefer to work for changes in the law rather than commit civil disobedience."

The bishops' conference has appealed to Secretary of State George P. Shultz to grant extended voluntary-departure status to Salvadoran refugees until conditions in their country become stabilized.

A resolution adopted by the Church of the Brethren last week made a similar plea but also endorsed sanctuary "as an appropriate Christian response" to the refugee problem.

So far, no church offering sanctuary has been approached by law enforcement agents.

Another factor in the churches' challenge to Reagan policy is the growing sense of solidarity with Christians south of the border.

U.S. Catholic churches have long had special ties with their counterparts in Latin America.

On the Protestant side, ties with "daughter churches" established through missionary efforts in Latin America have been supplemented by the founding last year of the Latin American Council of Churches, with representatives "from Pentecostals to Episcopalians," as Bolioli explained.

Appeals from its Latin American counterpart prompted the National Council of Churches' governing board, meeting in San Francisco last May, to adopt a statement on Central America. The delegates, representing 40 million people in member denominations, called for an end to U.S. military aid to Central American nations, U.S. support for negotiations in the Salvadoran conflict, United Nations involvement in "a peacekeeping role in Central America" and the provision of temporary legal asylum for Central American refugees.

Are the telegrams, the petitions, the letters, the visits to Washington producing any results?

"In Congress, yes," Bolioli said. "But at the State Department, we are politely received, we talk for an hour and a half. We move them--maybe one millimeter."