EDITOR'S NOTE - They posed as people of faith; one even wore a priest's collar. They were government agents, and their infiltration of the sanctuary movement led to the indictment of its leaders. This fourth and final part of a series, ''Seeking Sanctuary,'' explores ''Operation Sojourner,'' the government investigation that has become as much an issue as the movement itself.

PHOENIX, Ariz. (AP) _ The Rev. Jim Oines' Sunday night Bible study class at the Alzona Evangelical Lutheran Church used to be a popular weekly event for about 30 members of his congregation.

No one shows up anymore.

In January, half the class of mostly Central American refugees was arrested as ''unindicted co-conspirators'' by agents of the Immigration and Naturalization Service.

The arrests grew out of an undercover investigation into the sanctuary movement, an investigation that has touched off a growing rage among church leaders and legal experts who say this marks the first time the U.S. government has listened in on worship and planted agents in churches.

Critics say the government violated the First Amendment separation of church and state in its ''Operation Sojourner.'' Further, the investigation was pursued despite government assurances to Congress that churches would not be infiltrated.

''We have people now afraid to come to church because the person sitting next to them might be a spy,'' said Oines, who was not implicated in the investigation.

Through the 10-month course of ''Operation Sojourner,'' agents posed as sanctuary workers, dressed as clergy, taped meetings and religious services, recorded license plate numbers from a church parking lot and produced transcripts that contained not only talk of illegal aliens but also the words to hymns and prayers. No warrant was ever obtained by the INS.

The investigation resulted in the arrests of the aliens and the indictment of 16 sanctuary movement leaders. Three pleaded guilty in plea bargains, and charges against two were dropped.

''I was shocked. Every church person I know is quite surprised the government has taken this course,'' said Dr. William Thompson, former head of the General Assembly of the Presbyterian Church and now associate secretary general of the World Conference on Religion and Peace.

''We have canvassed American law and find nowhere where the government infiltrated the church,'' said the Rev. Robert Drinan, a Roman Catholic priest, a former congressman and now a law professor at Georgetown University. ''This has ominous implications.''

Defense attorneys for the sanctuary workers have focused on the government tactics, launched and directed by Phoenix INS agent James Rayburn. So far, U.S. District Judge Earl H. Carroll has agreed with prosecutors that the tactics were not overly intrusive and did not violate the rights of the defendants. Carroll also allowed evidence from the undercover operation to be used despite the lack of a warrant.

Attorneys say the tactics will remain a focus of the trial and may face later challenges in civil courts.

Defense attorneys have attacked the credibility of the two men hired to infiltrate the movement and introduce undercover INS investigators to sanctuary leaders. Both had been implicated in commercial smuggling operations before they went to work on Operation Sojourner, and on Oct. 1, prosecutors asked Carroll to remove from the witness list the name of one of two informants.

In pretrial filings, defense attorneys charged that Solomon Graham, who used the name Jose Morales, had provided prostitutes to migrant farm workers near Phoenix. Graham denied the allegation in the government's motion, but special assistant U.S. Attorney Donald Reno asked Carroll to prohibit defense attorneys from soliciting testimony about Graham's alleged association with prostitutes.

Oines' Bible study class had been electronically bugged by the other informant, Jesus Cruz, who attended about 15 sessions and supplied names of illegal aliens to government officials.

''I thought the whole purpose of the Constitution is to keep this kind of thing from happening so that churches don't have to live in fear,'' Oines said.

INS officials contend that undercover work is a routine and necessary part of immigration work.

''Informers are the normal course of events in smuggling cases,'' said INS spokesman Duke Austin.

The American Civil Liberties Union filed a brief with Carroll challenging the investigation, contending in part that ''pastors and parishioners are entitled to at least as much First Amendment due process as pornographers.''

''We think this has created a chilling effect on the practice of religion,'' said ACLU spokesman Wade Henderson. ''Some people may not be willing to discuss their innermost feelings in church.''

Congress has also questioned the investigation. Eight months into ''Operation Sojourner,'' INS Commissioner Alan C. Nelson responded to a letter from 10 congressmen who urged no prosecution of sanctuary movement workers. Nelson said the federal government would neither target the sanctuary movement nor raid churches.

''There will not be any special targeting of any particular individuals or groups for prosecution,'' Nelson wrote. ''Consistent with past and existing policy, INS ordinarily does not enter churches.''

Even as he wrote the letter, Nelson was aware of the Arizona operation, according to Austin.

''We didn't target the sanctuary movement,'' Austin said. ''We did say, 'Hey, there's a very active group in Phoenix.' That was a target effort against that group of what we would say are smugglers.''

Henderson wondered why the government went to such investigative lengths to indict people who had publicly announced how they intended to break the law, and often when and where. He said the ACLU didn't understand why the INS had not interviewed anyone or empaneled a grand jury to indict sanctuary movement workers, rather than set up a secret operation.

''Everything in the sanctuary movement has been done in the open,'' Henderson said. ''This was a very disturbing sting operation when it didn't have to be done to get the same information.''

Beyond all the arguments over constitutional rights, Oines says he just wishes he still had his Bible study class.

''There was a real need among these people for some kind of worship,'' he said. ''Now they don't have that.''