Fear of widespread defections from his Peoples Temple, and particularly the threatened loss of one 6-year-old child to two disillusioned former members, was one of the catalysts that drove the Rev. Jim Jones to order the mass suicide of his congregation 10 days ago, according to a well-informed source close to the temple.

The source, who has been in close contact with Jones and the temple over the last several years, says Jones was convinced the defections would fuel growing public criticism of his cult and bring on the demise of his 20-year-old movement. He ordered the destruction of his church, this source believes, as a final collective "punishment" for the "sins" of defectors who had "betrayed" him and turned against the church.

A central figure in this unfolding drama, according to this source, was John Victor Stoen, 6, whom Jones claimed he had fathered.

Stoen's parents, Grace and Timothy Stoen, had been highly respected members of the Peoples Temple. Jones considered Tim one of the most knowledged people about the inner workings of the church. After they left the temple in 1976 and 1977, the Stoens waged a bitter custody fight to regain their son from Jones, and were a major factor in prompting Rep. Leo Ryan's ill-fated fact-finding mission to Guyana this month.

For Jones the battle with the Stoens for custody of the child apparently became the ultimate symbol of his life-and-death struggle against defectors, with the very existence of his congregation hanging in the balance.

It was Jones' fear over a year ago that he was in danger of losing John - and the exposure that the Stoens' efforts to win him back would bring on the church - that prompted Jones' first threat to order mass suicide in his Guyana congregation, according to a former temple member and to temple attorney Charles Garry.

And 10 days ago, according to the source close to the temple, it was the Stoens' renewed attempts to win their child back that played a key role in triggering Jones' decision to self-destruct his church.

According to this source, a church doctrine required "group punishment" for the "sins" of those who threatened the cohesion of the group. (Errors deserving punishment committed by any member were considered errors committed by, and against, the congregation as a whole. To symbolize punishment of the group, Jones would sometimes be beaten before the individual offender.)

During Ryan's visit, it appeared that a number of church members, including young John Stoen, would commit the ultimate sin of of defection.

The ultimate punishment of the group was therefore deemed necessary by Jones who, according to this source, proclaimed the mass suicide the "will of the people" to self-destruct.

The Stoen family role in the evolution of the Peoples Temple dates to 1970 when Gracie Stoen joined her husband as a member of Jones' congregation in Ukiah, Calif. Timothy Stoen, then in his early 30s, rose quickly in the ranks, becoming the temple's prime legal counsel and a trusted adviser to Jones.

"I did nothing either with respect to the church or with respect to my own personal legal affairs without first consulting" Tom Stoen, Jones stated in a court affidavit unrelated to the custody issue early this year. "I am sure over the years he . . . gained more confidential information about Peoples Temple and its members than any other living person."

Grace Stoen, who was only 19 when she joined the church, also rapidly assumed a position of importance. She became a close confidence of Jones, and as the temple's "bookkeeper" gained an intimate knowledge of the complicated financial operations.

On Jan. 25, 1972, Grace Stoen gave birth to a son. John Victor Stoen was brought up in the temple, and raised by Grace, Jim Jones and other members of the congregation. In 1974 and early 1976, Grace Stoen signed documents, later ruled invalid, turning custody of her son over to the temple.

Grace, however, became increasingly disenchanted with Jones' operation of the church. In a court declaration in 1977 she charged that members were subjected to "beatings" and "public humiliation," and that Jones became consumed by a "paranoid world vision" and "claimed at various times to be the reincarnation of Buddha, Jesus Christ and Lenin."

"Thoroughly disillusioned," she stated, she "secretly departed" from the church in July 1976, leaving behind her son and husband. Four months later, Jones sent the child to Guyana.

After what she said were repeated unsuccessful efforts to persuade her husband and Jones to give back her son, Grace Stoen filed for divorce and custody of the child in San Francisco Superior Court in February 1977.

It was in the course of protracted legal wrangling over the case, according to Grace's attorney, Jeffery Haas, that Jones first made the allegation that he was the actual father of John Victor. The claim was heatedly denied by the Stoens, however, and was never made an issue in the custody proceedings.

On Aug. 26, 1977, Grace Stoen obtained a preliminary ruling from Superior Court Judge Donald King in San Francisco granting her custody of the child and ordering Jones' appearance in the court. Armed with a judge's order, attorney Haas says, he flew to Guyana within days to launch court proceedings there and secure John Victor's release from Jonestown.

It was Haas' appearance and initial success in the Guyanese courts in September 1977 that reportedly led Jones to issue his first threat to self-destruct his church.

Temple attorney Garry said in a recent interview that he was contacted at the time by the San Francisco temple office and was told that Jones had threatened mass suicide if the Stoens were not stopped. Garry said he linked up with Jones' wife, Marcie, in Chicago and "made a telephone radio patch to Jonestown."

"I told Jones it was madness," Garry recalled. "He said the people had demanded [suicide] and that he as their leader, had to give in."

Supporting Garry's account is a June 1978 sworn affidavit of Deborah Layton Blakey, the temple's former "finance secretary." Blakey, who was in the San Francisco temple office during the September "crisis," said Jones was bitter over Grace Stoen's defection and fearful of what Timothy, who was then also defecting, might say about the church. Jones "believed that he would be able to stop Timothy Stoen," Blakey said, "from speaking against the temple as long as the child was being held in Guyana."

With the arrival of attorney Haas at Jonestown, Blakey recalled, "the radio messages from Guyana were frenzied and hysterical." She and another temple member "were instructed to place a telephone call to a high-ranking Guyanese offical who was visiting the U.S. and deliver the following threat: Unless the government of Guyana took immediate steps to stall the Guyanese court action regarding John Stoen's custody, the entire population of Jonestown would extinguish itself in a mass suicide by 5:30 p.m. that day."

Both Garry and Blakey stated that after the suicide threat they tried to contact Guyanese officials to stop the court action. "Baiscally at that point," according to attorney Haas, "the court process shut down" and the Stoens' legal efforts to regain John Victor came to a standstill.

The Stoens turned to the State Department and members of Congress to put pressure on the Guyanese government and get the proceedings moving. Their efforts bore no fruit until August 1978 when Grace found a receptive ear in Congressman Ryan.

Haas says that Grace Stoen "met with Leo Ryan two or three times" to plead her case and describe her experiences inside Jones' church. She was "one of the central figures" in the California Democrat's ultimate decision to lead this fateful fact-finding mission to Jonestown, according to Haas. The Stoens also traveled separately to Guyana during Ryan's visit.

According to the source close to the temple, the threat that Ryan's mission would reopen the custody proceedings and force the release of John, plus Jones' fear of other defections to Ryan's contingent, triggered the "punishment mechanism" and the collective suicide in Jonestown.

Six-year-old John Victor Stoen is believed to have been among the victims of the mass poisoning. Grace and Timothy Stoen, according to Haas, are now in the San Francisco Bay area. They could not be reached for comment.