Among the mysteries that amaze Langhorne Motley, the departing assistant secretary of state for inter-American affairs, is why church involvement in politics is "a taboo subject. Nobody wants to touch it."

Motley tried to touch it the other day but missed. In a speech to the Overseas Writers that was reported in The Washington Post by Joanne Omang, Motley said, referring to Latin America, that it is "time to take politics out of the pulpit and the pulpit out of politics . . . The pulpit is misused when devoted to secular political causes." Not a namer of names but an able dispenser of innuendos, Motley lectured that "religious persons should not use the credibility they enjoy to market their personal, philosophical and political beliefs."

In other words: Shut up. Leave the Reagan policies in Latin America to experts like Motley.

This latest assault, like earlier ones, is misdirected. American church leaders who have been criticizing the Motley crew in Central America have earned the right to be heard. It is their personnel -- nuns, priests, brothers and laypeople -- who have been killed or harassed by government police or death squads.

Two of the four churchwomen murdered in El Salvador in December 1980 were under the sponsorship of Archbishop James Hickey, then of Cleveland. When Hickey, now the archbishop of Washington, testified last April before a House subcommittee on Latin America, it wasn't as a think-tank dilettante. He was a Catholic leader speaking of Catholic missionaries in Catholic countries. Who else should speak out on that topic? Roberto D'Aubuisson?

In the last three years, Catholic officials have spoken before at least five congressional committees. They are unbending in their belief that military aid is useless and wrong. With a new age of martyrdom now the reality in countries like El Salvador and Guatemala, the church's appearances in political forums have been paid for in blood.

Motley cheapens religion by claiming that church leaders are seeking "to market" their politics. Marketing is what the administration was up to in April when the president tried to sell the preposterous notion that Pope John Paul II "has been most supportive of all our activities in Central America." The Vatican quickly exorcised that idea from the public record.

Reagan's deception was a hustle for votes, occurring when he was asking Congress for $14 million for the contras in Nicaragua. This was opposed by the U.S. Catholic bishops. Only weeks before, Reagan had claimed that the Bible supported his position on MX missiles. A logical pattern had developed: Since God favored the nukes, of course the pope liked the contras.

Motley's attack on a rebellious clergy coincides with the administration's attack on churches that give sanctuary to Central American refugees. In Phoenix, paid government informers infiltrated religious services and Bible study classes to finger churchworkers who were helping homeless and penniless escapees from government-supported terror. One federal sneak secretly tape-recorded meetings in which nuns, ministers and priests spoke of their plans. Twelve churchworkers are now on trial for aiding illegal refugees.

Only this administration could see a Bible study class as a crime ring worthy of a bust by covert operation. An interviewer for the National Catholic Reporter asked the Arizona director of the U.S. Immigration and Naturalization Service if its invasion of churches violated the First Amendment rights of the defendants. "No," came the answer, "because our criminal investigation had the full approval of the U.S. attorney's office."

One of the 12 churchworkers under indictment (all face possible long prison sentences and fines) is Sister Darlene Nicgorski, who has worked among the poor in Mississippi, Guatemala and Mexico. Her explanation for her involvement in the sanctuary movement is something that Langhorne Motley needs to meditate upon, now that he is out of the government and still doesn't understand the church in Latin America: "The protection of life is a religious activity. As a School Sister of St. Francis, I am committed to giving, healing and defending life. . . . Religion is not just saying prayers and singing songs. The government can't tell us which people we can respond to and which ones we can't."

A similar response comes from John McAdoo, an official of the Unitarian Universalist Service Committee. This is the Boston-based membership organization that has led 10 congressional delegations to Central America since 1978. "I'm sure that the churches will be happy to get out of Latin American politics," McAdoo says, "just as soon as the administration stops distorting democratic values there and begins to create a consistent policy for peace and justice which doesn't resort to military aid."

Before Motley's homily, Alexander Haig, Jeane Kirkpatrick and George Bush took turns questioning the motives of church activists in Latin America. A choir appears to be forming, part of the Republican Gospel Hour.