A century and a half after its founding by a poorly educated, mystically inclined New York farmer named Joseph Smith, the Mormon church stands at the threshhold of great worldly success -- a wealthy, powerful institution moving quickly toward its goal of becoming one of Christianity's major denominations.

Yet, as thousands of the faithful flock, here this weekend to mark the 150th anniversary, some Mormons worry about the course their church will take in the midst of prosperity, increasingly ungovernable membership growth, and a swirl of vital, secular influences.

"There is growing dissent within the church," says J. D. Williams, professor of political science at the University of Utah and great-grandson of Brigham Young, who led the Mormons to this desert mountain valley in 1847. "The church finds itself increasingly fighting a rear guard action against everything that has happened over the last 40 years."

The Equal Rights Amendment, which the church stridently opposes, has been the most heavily publicized of the issues dividing church members. Last year Mormonism was rocked by the excommunication of Virginia housewife Sonia Johnson, an ERA activist.

While taking issue with some of Johnson's anti-church pronouncements, Williams and others in the small but influential liberal minority here see in her fate a portent of an increasingly authoritarian, right-wing trend within the church.

Much of the concern felt by Mormon progressives focuses on Ezra Taft Benson, 80-year-old president of the Quorum of the Twelve Apostles, the church's supreme administrative body, and heir-apparent to the mantle of president Spencer W. Kimball, the frail 85-year-old titular head of the church, recognized prophet, seer and revelator.

In a speech this year, Benson, Dwight Eisenhower's secretary of agriculture for eight years and long a supporter of right-wing causes, openly suggested that the prophet would be within his rights to use his exalted position to speak out on political matters, notwithstanding established church policy to the contrary. He also said the word of the church president should take precedence even over scripture or the teachings of "dead prophets" such as Joseph Smith.

"We are seeing now with Ezra Taft Benson the blueprint for a return of Old Testament theocracy," Williams said. "Benson is a messianic personality. This church is in great danger of falling under the spell of political and religious fundamentalism."

Church leaders are openly disdainful of Williams and other liberal Mormons who are, even by their own estimation, a small minority within Mormonism. Establishment spokesmen claim that the converts whom their conservative stand on ERA has won for the church far outnumber the birthright Mormons it has driven away.

"We feel ERA would not be in the best interest of the world," said N. Eldon Tanner, 81-year-old first counselor to President Kimball. "The reaction of those who are opposing us is of no concern.I am so confident that there's a majority of women in the church who agree with us."

For proof, Tanner and other likeminded Mormon leaders point to the church's booming membership rolls. The Church of Jesus Christ of Latterday Saints, as Mormons call themselves formally, has grown by over 30 percent in the last five years, to well over 4 million faithful worldwide. Much of this growth has taken place in such countries as Mexico, Korea and Somalia, where church missionaries, who now number 30,000 around the world, have been successful in selling Mormonism's family-centered and rigidly puritanical gospel.

At the same time the church has continued to amass immense wealth, much of it from the tithe, the regular donation of 10 percent of income by loyal members. The church also controls a vast business empire that included apartment buildings in New York, a huge 60-store shopping mall in Salt Lake City, ranches, an industrial park, two insurance companies, a publishing firm, a newspaper, television and radio stations and even a village in Hawaii.

All these sources bring into the church exchequer in Salt Lake City an annual income estimated at as much as $1.3 billion annually. Much of this money is used to build new chapels and temples and to support the church's massive media operation which includes sophisticated television satellite hookups, mass mailings and even a movie studio at Brigham Young University in Provo, Utah.

Through the electronic media, the church beams its message to millions of people on every continent, said Heber Wolsey, head of the church's Public Communications Division. While shying from politics, the church will continue to push its positions on such "moral issues" as abortion, pornography and the ERA, Wolsey said.

(The church opposes the ERA on the basis of morality, fearing that the rights extended to women would end up sanctioning such things as abortion and homosexual marriages, both abominations in the eyes of church authorities. In addition, the church believes the amendment would undermine women's traditional role in the home, a cornerstone of the family-oriented faith.)

Some church elders, including Tanner and Wolsey, admit there is apprehension among some Mormons about the possible assumption of the prophet's mantle by the highly politicized Benson. But Wolsey is not worried, he said, because "the spirit of the Lord is sure to temper him."

Such faith in the direct interest of the deity in the affairs of the Mormon people has from the beginning been a central tenet of the church. Founder Smith preached that he himself had spoken directly to the angel Moroni, a messenger of God, who led him to the book of Mormon, the faith's basic scripture.

In 1827 Smith claimed Moroni told him to climb Cumorah hill, outside present-day Palmyra, N.Y., where, as the angel instructed, he unearthed the gold plates which, with divine inspiration, he later translated into the book of Mormon. The book tells the history of a purported offshoot of some Israelite tribes who journeyed to America in the wake of Jerusalem's destruction at the hands of the Babylonians six centuries before Christ.

In the new world, the former Israelites built a civilization, according to the book, and were later visited by Christ following his resurrection. Ultimately these wanderers split into two nations, one ultimately destroyed by the other. The remnant nation faded into oblivion sometime in the 4th century A.D. Their descendants, Mormons believe, are the Indians of both North and South America.

Mormon, revealed to Smith as one of the last great prophets of these dying American remnants of Israel, left the plates in the ground until Smith was visited by moroni, the resurrected son of Mormon. The plates are said to have been returned by Smith to Moroni. Their authenticity has been repeatedly challenged by both scholars and those who ridicule Mormon beliefs.

From the beginning, Smith's new faith became a target of persecution, both by other Christian groups and later by splinter sects of Mormonism itself. Shortly after the church was founded -- 150 years ago today -- by Smith and five followers at the Peter Whitmer farm near Fayette, N.Y., the Mormons were chased across the continent, finally settling in Illinois, where they founded the city of Nauvoo.

At all times, and even today, the Mormons have been viewed as a peculiar people with strange religious rites, secret temple ceremonies and garments, and an unquestioning faith in their leader and prophet. By 1844 the Saints, as they like to call themselves, had made Nauvoo into a sort of Zion, with a population numbering 20,000, dwarfing at the time the then fledgling city of Chicago.

But Smith's followers, though armed and well organized, were not safe even in Nauvoo. While attempting to run for president of the United States in preparation for the building of his kingdom of God on earth, Smith was arrested by Illinois authorities. He was assassinated on June 27, 1844, at the age of 39 in his jail cell at Carthage, Ill., by an anti-Mormon mob.

Following his death, the church was led by Brigham Young, who led the Saints across the plains in 1847 to this vast mountain valley, then totally desolate. Here the Mormons were free to build their Zion in earnest -- and even practice polygamy until 1890. Today most Mormons, even many of the dissenters, cherish their forebears' dramatic history. They speak with reverence of Smith's highly optimistic teachings that men are indeed perfectible beings. "As man is, God once was; as God is, man may become," Smith preached.

The influence of the ideal is most strongly felt here in Utah, the home of one million Mormons, 70 percent of the state's population. Salt Lake City, although a metropolis of over 500,000, remains a clean, quiet town -- ideal for families -- with a low crime rate and low unemployment. In some Utah locales, particularly in the countryside, it is still a feat for a stranger to find a place to buy a drink.

Through strong conditioning, Mormon youths are encouraged to attend school, marry, and have children. They tend to have few delinquency problems and take school seriously. (Utah's per capita expenditure for public education is the nation's highest.) Mormons as a group are remarkably healthy. Recent studies show a far less incidence of cancer than non-Mormons.

For many of the 14,500 people who attended the church's commemorative ball here Friday night -- lemonade was the strongest drink -- and for the 9,000 who packed the Tabernacle today to hear President Kimball, these statistics are ample proof of the righteousness of their faith and their evangelical fervor.

Yet for some Mormons this well-ordered, disciplined, family-centered way of life can be very constraining. Dissenting Mormons, many of them young and well educated, see the church as a tired bureaucracy led by men in their 80's totally out of touch with modernity.

"They need to ease up a little," said a Utah state official. "They have to allow for pluralism. They have to let people like me breathe."

This prominent Mormon, who insisted on anonymity to protect his career, is saddened by what he claims is growing bureaucratization of his church.

Some Mormon activitists, particularly pro-ERA women, are banking on the church's eventual willingness to change its policies to save its own growth in a modern society. They point out that the church in June 1978 overturned a long-standing policy and opened its priesthood to black males. Women are still excluded.

"Who's to say I can't be a Mormon because I don't do the stereotype things," said Marilee Latta, a Salt Lake City public relations consultant and a leader of the Utah chapter of Mormons for ERA. "That's the only way for the church to grow. To start opening up to women, to new ideas. We can't be a bunch of robots."

Latta said she prays that the church will not select Benson as Kimball's successor and will instead select "a younger prophet" to take the church through the difficult years.

Most observers, however, believe such a move is virtually inconceivable for such a tradition-bound institution.

Caught between societal pressures to reform and their staunchly conservative convictions, some thoughtful Mormons believe that, with Benson's elevation inevitable, the church could be lurching toward a major schismatic crisis. "It would be too bad if this uptight, conservative trend, continues," said Salt Lake City Mayor Ted Wilson, a prominent Mormon liberal. "If the church stays closed, it will be like steam collecting inside a kettle. It could blow up someday."