The Rev. Chrysostomos Economakos listened solemnly as the young prisoner confessed his sins, then gave him holy communion. Minutes later, as they chatted at the Metropolitan Corrections Center here, William P. Kampiles, 23, blurted out his exasperation at the charges against him.

"I can't tell you what he said at first; it was a confession," the Greek Orthodox priest recounted. "But after he took holy communion, he said to me, 'It's not true. It's not true. I'm not guilty.'"

Those who should know Kampiles best - priest, family, friends - say they believe him. The government of the United States does not. It is prosecuting him for espionage - a crime, that, on paper, carries the death penalty - and it has compiled what FBI Director William H. Webster, for one, considers "a very convincing case."

Specifically, Kampiles, a former CIA "watch officer," has been accused of selling one of the nation's top secret, as technical manual describing the capabilities (and thereby the limitations) of the KH-11 spy satellite, to the Russians for a mere $3,000.

A sophisticated successor to the original "Big Bird" satellite the new KH-11 spy in the sky is so sensitive that it reportedly can capture details less than a foot long from altitudes of 100 miles, transmitting the photographs back to earth in digital computer code.

"To us, it's unbelievable," Ecanomakes said of his parishoner, a one-time altar boy on Chicago's Southeast side. "He was always a very quiet boy, coming to church, receiving communion. Among the Greek community, he was one of the best . . . You can't believe a person could lead such a double life."

The ingredients of the government's case include an apparently compromising letter Kampiles wrote to a CIA colleague last May and reported admissions to the FBI in August that he had indeed taken the document home with him one day last year and delivered it months later to a Soviet agent.

Although it is not admissible at the forthcoming trial, according to court records a polygraph test also was administered, with apparently unfavorable results. His lawyer, Michael D. Monico of Chicago, has filed a sealed motion in federal court asking for a copy of the polygraph itself.

The Justice Department, meanwhile, is casting about for some constitutional way to prosecute the case in open court without making public the KH-11 manual, the key piece of evidence. Now that the Soviets have it, the government is striving vigorously to prevent further dissemination, even if that means something less than a genuinely public trial. Defense attorney Monico is under court order to keep his notes in a 600-pound "General-Services-Administration-Approved Security Container," installed in his law office this month.

Despite all that, the actual damage done - beyond exposing the CIA as a place where secrets can march out the front door without anyone's noticing - is a matter of some dispute within the intelligence community.

Some sources call it "a real disaster" that could enable the Russians to hide their missile sites from detection. Others suggest the manual is just as likely to demonstrate to the Soviets how hard it is to avoid effective monitoring, even without a strategic arms limitation treaty. The truth may lie in between.

"It's significant . . . it's not trivial," Webster said of the loss at a luncheon meeting with Washington Post editors and reporters. But he added, "I'm not wringing my hands about it."

The case has also fueled fresh gossip about the possibility of a "mole" or master spy for the Russians, for whom Kampiles is simply the fall guy, a sacrificial offering to distract investigators.

"The Kampiles case," former CIA director Richard M. Helms asserted to New York magazine recently, "raises the question of whether or not there has been infiltration of the United States intelligence community or government at a significant level."

Thus far, however, the only "evidence" of the mole's existence is the fact that he has never been caught. "If you wanted to coin a phrase, you could call it Angletonitis," said one source, alluding to the suspicions long held by former CIA counterintelligence chief James J. Angleton, who is now "out in the cold" after a long career at the agency. "This is something people have been looking and looking and looking for since the agency was established."

Others sources have suggested that the dispute is a reflection not of any real evidence from the Kampiles case, but rather of a bitter ideological "battle of the spooks," the outs vs. the ins, that has been raging for several years, relying on current events for ammunition. Angleton, for instance, is known to believe that the CIA's counterintelligence efforts wer "emasculated" following his ouster in December 1974.

In any event, both FBI Director Webster and CIA Director Stansfield Turner say they have seen nothing to support the "mole" theory.

"I have no knowledge or indications that there is a mole, a secret agent, inside the Central Intelligence Agency," Turner said recently on CBS' Face the Nation. "But for me to sit here and tell you that I was utterly confident that there was none would be foolish."

Going a step further, Webster told The Post he had no information that the "mole" talk was well-founded "or even likely to be so."

Indicated by a federal grant jury in Hammond, Ind., where he is scheduled to go on trial Nov. 6, Kampiles stands accused of selling the "Top Secret" KH-11 manual on a trip to Athens, Greece, last winter. He allegedly turned it over to a Soviet agent named Michael at the Russian embassy in two segments, the first few pages on Feb. 23 and the rest on March 2. According to one account, Kampiles even signed a receipt for the $3,000 that "Michael" finally gave him.

The CIA didn't realize the document was missing until months later. An investigation was triggered when Kampiles wrote a former colleague at the CIA, in May, to say that he had been in contact with the Russians and that they wanted him to obtain information for them. Kampiles offered to supply "disinformation" instead, but the CIA's suspicions were aroused when he repotedly told his colleague that the Soviets had already given him some money.

The soviets, according to U.S. intelligence analysts, are not in the habit of paying out cash without receiving something in return. The FBI was called in Kampiles was arrested Aug. 17 at an apartment he had been sharing with a friend in Munster, Ind., for several weeks.

The son of Greek immigrants, Kampiles grew up in a predominantly Polish, Slovak and Greek neighborhood called Hegewisch in Chicago's Southeast corner, an improbable breeding ground for spies.Oldtimers still remember, with precision, how many boys they sent off to fight World War II. The main drag, South Baltimore Avenue, is lined with neighborhood pubs and mom-and-pop stores.

Kampiles grew up in a small apartment on Burley Avenue with his mother, Nicoleta, and his brother, Michael. Their father, Peter Kampiles, a steelworker at Republic Steel's nearby plant, died of cancer in 1964. Mrs. Kampiles took a job in the cafeteria at Ford Motor Co.'s huge assembly plant in the same neighborhood. At 63, she still works there.

"My brother identifies himself as an American," Michael Kampiles, 25, said the other day in a tremuously earnest voice. "He loves his country. He's conservative politically . . . yes, my-country-right-or-wrong. I believe him to be innocent."

Like his older brother who still lives at home, William Kampiles became an altar boy at age 6 at the Church of the Assumption a block away. He also learned how to make money at an early age, working a paper route, delivering groceries, driving a cab, working at a steel mill, putting himself through school. A 1976 graduate of the University of Indiana in Bloomington, he got a job as a hospital supply salesman - "the No. 1 salesman in his territory," his brother says - until the CIA called, following up on a college interview.

Kampiles joined the agency in March 1977 at less than $15,000 a year and was assigned to the CIA's round-the-clock Operations Center as a "watch officer," monitoring the constant cable traffic from overseas, but he left less than eight months later, reportedly frustrated in his hopes of becoming a case officer. Shortly before he left, he allegedly took home the Operations Center's copy of the KH-11 manual from a file cabinet.

Some CIA veterans are wondering darkly who assigned Kampiles to the Operations Center, as though his alleged filching of the manual were part of some conspiratorial design right from the outset. But other sources say the place, rightly or wrongly, has been used as a training ground for junior officers for years.

Through his lawyer, Kampiles is saying he didn't sell the manual to the Russians. And his lawyer says he has a strong case to present. Sources close to Kampiles indicate that he has no quarrel with the letter to his CIA colleague and is currently seeking a witness from Athens, a woman, who might support the notion that the Russians did pay money to enlist him without getting anything in return.

Defense attorney Monico has also asked the government in formal pleadings for any information indicating that the Soviets may have learned of the KH-11's capabilities before Kampiles's alleged sale of the manual. And he has demanded "any information regarding the possibility that the Soviets 'burned' defendant Kampiles in order to provide the United States government with a plausible explanation for the loss of the KH-11 document, thereby protecting a high-placed agent of the Soviets who had penetrated the Central Intelligence Agency."

The requests are not likely to be productive in light of all the high-level declarations that there is no such information to be had. David T. Ready, the U.S. attorney for northern Indiana who will prosecute the case (and will not seek the death penalty), foresees no startling revelations, at least not by the government.

"I don't expect any surprises," Ready says of the trial ahead. "I don't think Washington is holding back on me. They'd better not be."