Officials of the National Institute of Mental Health were astonished when a group of young demonstrators showed up to protest a meeting on the subject of religious cults such as the Unification Church and the Scientologists.

The Monday meeting at the NIMH staff college in Rockville was closed. Notice of it had never been posted. Most of those at the meeting were said to be critical of cults and the matters to be discussed were regarded as "delicate."

Dr. Harold Goldstein, assistant chief of the NIMH staff college, told one person privately that he felt a more public session, with pro-cult spokesmen present, might simply "erupt."

"It's like Arabs and Jews - the two can't sit down at the same table," Goldstein was quoted as saying. No one could figure out how the demonstrators had learned the time and place of the session.

The pickets, one in a Nazi-type uniform with swastika to demonstrate "distaste for fascism," milled around for several hours to protest "psycho-surgery" and "electroshock therapy" and to demand: "Get the government out of mind control." The pickets identified themselves as from the Citizens Commission on Human Rights (a group started by the Church of Scientology but now sponsored by the Association of Scientologists for Reform).

The meeting was cut short after the Office of General Counsel fo Health, Education and Welfare reversed an earlier ruling and held that a closed meeting wasn't allowed without advance notice.

HEW officials were perplexed at how the demonstrators and two reporters for the Unification Church newspaper, New World, had learned of the meeting.

One HEW official mumbled darkly that it seemed to confirm rumors that the "Moonies," Scientologists and other cults have spies everywhere. "Maybe the [Unification] church has infiltrated the place," he muttered.

Actually, it wasn't that mysterious.

It all goes back to a June 5 meeting of the National Conference of Christians and Jews, held at New York City's Queens College, on youth cult groups.

Kathie Lowrey, public affairs director of the Unification Church (Moonies), says she was wandering through the crowd when she heard a participant, Dr. Lester Rosenthal, say that federal funds might soon be available for "rehabilitation" of Moonies and other cultists.

She listened to some others and soon learned that a June 25-26 closed meeting on cults - featuring a large group she described as critical of cults - was to be held at NIMH.

Among those invited, she said yesterday, were Galen Kelley, whom she described as a "deprogrammer" of cult members; Gary Sharff, former Unification Church member and also a deprogramming expert; Margaret Singer of the University of California; Dr. John Clark of Harvard; Hardat Sukhdeo of the New Jersey Medical College, and Marc Galanter of Albert Einstein College of Medicine in New York.

Lowery said she was extremely distressed when she heard about the meeting. She said it appeared to be an attempt to gather, behind closed doors and at government expense, people who were mostly associated with a very strong bias against "new religions."

She promptly wrote a letter to this effect to NIMH and soon received a phone call from Goldstein. He inadvertently revealed, that the meeting would be held at the staff college and volunteered to convene a sepparate meeting soon to hear the views of those more favorable to "new religions."

Lowrey notified the church's lawyer, Jeremiah Cutman of the American Civil Liberties Union. She also revealed the time and place of the meeting to the Unification Church's New World and a D.C. group called APRIL - Alliance for Preservation of Religious Liberty. It, in turn, told the Citizens Commission on Human Rights.

So: No spies. No infiltration. Simply one attentive woman at a Queens College conference.

Despite it all, HEW officials said yesterday that NIMH may try to hold yet another closed meeting, this time after giving the requisite notice.