Although the use of undercover agents has become as American as light beer, until now no government agency has admitted to planting them in a church. In trying to make a case against sanctuary workers in Arizona, however, the U.S. government conducted a 10-month undercover investigation, Operation Sojourner, which led to the indictment of 16 people (later reduced to 12). The charges include bringing undocumented aliens illegally into the United States and then concealing them.

The government's case is based on about 100 covert tape recordings, many of them made in church, by two paid government informants, who had disguised themselves as ardently religious supporters of the refugees. Among the tapes are Bible study classes and prayer services.

During recent pretrial hearings in Phoenix, U.S. District Judge Earl Carroll has not appeared particularly impressed with defense arguments based on the state of human rights in Central America or the notion that international law can transcend American immigration rules. Judge Carroll has, however, been disturbed by testimony about the government's creation of a new frontier for undercover agents. Sending "people paid to do it and wired to do it into places of religious activity," the judge said, means "the whole process has been sullied in a sense."

An argument is being made by lawyers for the American Civil Liberties Union and the Center for Constitutional Rights, among other attorneys involved in the case, that all the "sullied" evidence obtained by the false congregants should be thrown out.

The question, they say, is not whether the government can never send an informant into a church. Crimes can be flaunted in holy places, as is richly evident in the histories of England and Russia. But in this country, can covert agents constitutionally be sent into a church without first going before a neutral magistrate -- a judge who hears the government's probable cause for planting the informants and then decides whether the government has shown compelling, specific reason to compromise the holy place?

As it is now, no undercover agent, whether he slips into a church or a congressman's office, needs a warrant. Responsible for this largest hole in the ever more tattered fabric of the Fourth Amendment is the Supreme Court, which has never understood that a covert informant is far more intrusive than a wiretap or bug.

The constitutional significance of the sanctuary workers' case is that it may finally produce some warrant requirement for undercover operatives, at least in a church setting. At issue and at risk are First Amendment protections for free exercise of religion and, within that context, for freedom of speech and association. The informants, it should be kept in mind, did not tape only meetings in which those "conspiring" to smuggle aliens were present. They picked up the conversations of a lot of other church members. Yet, only the government handlers of the government informants decided what was to be taped and when. No detached magistrate was supervising Operation Sojourner.

The defense maintains that the warrant clause of the Fourth Amendment must be invoked whenever the government intends to use undercover informants in ways that may threaten significant First Amendment values. Like sending them into a church to pick up anything they can.

During the pretrial hearings in Phoenix, Pastor Eugene Lefebvre of the Sunrise Presbyterian Church testified that a woman who took part in a church discussion that was later found to have been surreptitiously taped is now afraid that the FBI has opened a file on her and that she could be targeted when she applies for a teaching position.

And James Oines, pastor of Alzona Lutheran Church, said from the stand that he no longer holds Bible study classes because some members of his congregation are afraid to come to the church. They no longer have faith that the person sitting next to them is revealing his true heart.

Oines added: "The deepest aspect of their faith and trust was violated. It turned out that we were as gentle as doves but not so wise as serpents."

Among the defendants are a Protestant minister, Roman Catholic priests and nuns, a social worker, a college student and a Quaker rancher. The lawyer for one of them, James Brosnahan, told the National Catholic Reporter that "the government has not made a practice of invading church buildings to apprehend people. I would like to think of it as an aberration that will never happen again."

It all depends on what the courts say. This, after all, is an administration convinced that God, being on its side, would not consider an undercover agent to be trespassing in one of His churches under these circumstances.