SALT LAKE CITY (AP) _ Jeff Decker was just one of hundreds of young Mormon missionaries troubled by the news that a charismatic American preacher was on his way to Chile to undo their work.

But Decker's was an especially painful dilemma. The preacher, who had made a career of ridiculing the Mormons' most sacred rites, was his father.

While Jeff Decker was busily bringing souls to Mormonism during his two- year mission in Chile, his father, Ed Decker, was becoming one of the best- known opponents of Mormonism.

The elder Decker, a former Mormon Sunday school teacher, is the founder of Saints Alive in Jesus, an organization based in Issaquah, Wash., and dedicated to converting Mormons to fundamental Christianity, or at least foiling their enormous missionary program.

His two mocking films, ''The God Makers'' and ''Temple of the God Makers,'' have outraged church members and even angered some of his colleagues in other anti-Mormon ministries.

Decker, 50, steadfastly defends his work, claiming it has saved 10,000 to 15,000 people from what he considers a non-Christian cult.

But Decker's ministry to the Mormons has created rifts he has found difficult to mend. He concedes he has failed to reach the Mormon community he is trying to save.

And he and his son were able to enjoy a family relationship only after declaring a truce on the subject of religion.

''I don't think my father is an evil man, but I think he is in error in what he is doing. I think he was led astray,'' said Jeff Decker. ''He looks at me like a lost son, out on a cliff. I look at him the same way.''

Reared in a family of lukewarm Episcopalians, Decker joined The Church of Jesus Christ of Latter-day Saints in 1961 after five years of marriage to a Mormon convert.

Decker said his happiness with Mormonism waned when he began socializing with a group of Protestants critical of Mormon doctrines. He could not accept the church's premise that its founder, Joseph Smith, was called by God to restore pure Christianity to the Earth. He also struggled with the Mormon precept that righteous humans can become gods and goddesses after death.

Decker divorced in 1969 but continued to attend Mormon services periodically. Six years later, at a Protestant revival, he said he knelt at an altar and asked God for salvation.

''I got up off the altar floor and felt 100 pounds lighter. I felt just great,'' Decker said.

By the end of the year, he had asked to be excommunicated from the Mormon Church. In 1978 he organized Ex-Mormons for Jesus, which later became Saints Alive in Jesus.

Decker's critics contend he is prone to exaggeration. In an interview, he claimed to have shattered the church's strength in the Pacific island nation of Tonga last summer.

''I think the work we did there in Tonga has totally put an end to Mormonism there,'' he said. The church counters that it recorded 1,454 convert baptisms in Tonga in 1984 and 806 in the first six months of 1985.

Decker and his son also give differing accounts of his visit to Chile. Decker claims church officials refused to tell him his son's whereabouts, then gave him a false address. He said he was allowed to see his son only after he threatened to call police.

Jeff Decker said his father was accidentally given the address of another missionary in a village 20 miles away.

''When Dad came down, I didn't have any desire to see him as a preacher, or in terms of what he was doing,'' he said. ''I wanted to see him as my father, not the Reverend Decker.''

His ex-wife, Phyllis Danielson, believes there was more to Decker's entry into the ministry than religious zeal. His oft-stated goal during their marriage, she said, was to earn an annual income equal to $1,000 for each year of his age.

''Mr. Decker wanted power and money'' when he turned from convert to critic of Mormonism, she said.

Decker admits that until he was ''born again,'' money was a major motivation in his life. But that ended, he said, with his experience at the altar. He said he reported an income of $10,000 on his 1984 federal tax return.

Saints Alive is financed by donations, public speaking fees, sales and rentals of Decker's films, and royalties from his writings, including a book version of ''The God Makers'' that sells for $7 in paperback.