The trouble is, it isn't true.
Today, The Church of Jesus Christ of Latter-day Saints has more than 12 million members on its rolls, more than doubling its numbers in the past quarter-century. But since 1990, other faiths - Seventh-day Adventists, Assemblies of God and Pentecostal groups - have grown much faster and in more places around the globe.
And most telling, the number of Latter-day Saints who are considered active churchgoers is only about a third of the total, or 4 million in the pews every Sunday, researchers say.
For a church with such a large, dedicated missionary corps constantly seeking to spread its word, conversion numbers in recent years tell an unexpected story.
According to LDS-published statistics, the annual number of LDS converts declined from a high of 321,385 in 1996 to 241,239 in 2004. In the 1990s, the church's growth rate went from 5 percent a year to 3 percent.
By comparison, the Seventh-day Adventist Church reports it has added more than 900,000 adult converts each year since 2000 (an average growth of about 5 percent), bringing the total membership to 14.3 million. The Assemblies of God now claims more than 50 million members worldwide, adding 10,000 new members every day.
Russia provides a dramatic example of different religious growth rates. After more than 15 years of proselyting there, LDS membership has risen to 17,000. During the same period, Jehovah's Witnesses membership has increased to more than 140,000, with some 300,000 individuals attending conferences.
Graphing activity: When the Graduate Center of the City University of New York conducted an American Religious Identification Survey in 2001, it discovered that about the same number of people said they had joined the LDS Church as said they had left it. The CUNY survey reported the church's net growth was zero percent. By contrast, the study showed both Jehovah's Witnesses and Seventh-Day Adventists with an increase of 11 percent.
"Because membership statistics are prepared and reported differently by various religious groups, the LDS Church does not publish comparisons of total membership to other faiths," said LDS spokesman Dale Bills on Friday.
On the question of how many Mormons are actively participating, Brigham Young University demographer Tim Heaton noted in the Encyclopedia of Mormonism that attendance at weekly sacrament meetings in the early 1990s was between 40 percent and 50 percent in Canada, the South Pacific, and the United States. In Europe and Africa, the average was 35 percent. Attendance in Asia and Latin America hovered around 25 percent.
By multiplying the number of members in each area by these fractions, David G. Stewart Jr. estimates worldwide activity at about 35 percent - which would give the church about 4 million active members.
Stewart, an active Mormon who served a mission to Russia in the early 1990s, has been conducting research on LDS missionary work in 20 countries for 13 years, examining census figures, and analyzing published data.
Take Brazil. In its 2000 Census, 199,645 residents identified themselves as LDS, while the church listed 743,182 on its rolls.
"There may be any number of reasons for the discrepancy," Bills said, "including personal preferences of some citizens regarding disclosure of their religious affiliation."
Retaining members: Stewart says Mormons need to be aware of such statistics to be more effective missionaries. To that end, he is publishing his research, along with a description of what he calls "tested principles to improve growth and retention," in a forthcoming book, The Law of the Harvest: Practical Principles of Effective Missionary Work.
"It is a matter of grave concern that the areas with the most rapid numerical membership increase, Latin America and the Philippines, are also the areas with extremely low convert retention," says Stewart, a California physician. "Many other groups, including the Adventists and Jehovah's Witnesses, have consistently achieved excellent convert retention rates in those cultures and societies. Latter-day Saints lose 70 to 80 percent of their converts, while Adventists retain 70 to 80 percent of theirs."
Perhaps the best measure of LDS Church growth is the rate of new church units, such as wards (congregations) and stakes (like a diocese). Because they are staffed by volunteers, such units cannot function without enough active members.
In 1980, The Ensign, the LDS Church's official magazine, predicted that membership would grow from 4.6 million members at that time to 11.1 million members in 2000, and from 1,190 stakes to 3,600 in 2000. While the number of members came very close to the projected value, there were 2,602 stakes worldwide at the end of 2002.
"You can use these trends to say that the percentage is slowing, the numbers have leveled off or they are dropping. They tell us what is happening right now," Heaton says. "But to try and tell us about the future is risky business. What if all of a sudden China or India lets us in and the [missionary] work takes off?"
Predicting the future: In 1984, University of Washington sociologist Rodney Stark was astonished to discover that the LDS Church's growth rate from 1940 through 1980 was 53 percent. He estimated that if it continued to grow at a more modest 30 percent, there would be 60 million Mormons by the year 2080; if 50 percent, the figure would explode to 265 million.
He famously predicted that the LDS Church "will soon achieve a worldwide following comparable to that of Islam, Buddhism, Christianity, Hinduism, and the other dominant world faiths."
Latter-day Saints were on the threshold of becoming "the first major faith to appear on earth since the Prophet Mohammed rode out of the desert," Stark wrote.
Many people, especially Mormons, eagerly embraced Stark's assessment. In recent years, though, some scholars have challenged his assumptions.
For one thing, True Pure Land Buddhism, Sokka Gakkai, Baha'i and Sufism are all of comparable or greater size and have arisen since Islam in the 7th century, said Gerald McDermott, a religious studies professor at Virginia's Roanoke College who gave a paper at a Library of Congress symposium on Mormonism in April.
One key to Mormonism becoming a world religion, McDermott says, is how well it can transcend its founding culture to become universal. Catholicism, for example, began in Jerusalem but found a home in many other places, where it easily assimilated into local cultures.
The LDS message has found a ready audience in Latin America and the South Pacific, where Mormon missionaries can tell people God did not neglect them. The Book of Mormon is the story of a Hebrew family that migrated from Jerusalem to the New World and tells of a visit to their descendants by Jesus Christ after his resurrection.
Still, the church may not fare as well as other Christian religions in Africa and China, since it has no such reassurance for them, he says.
American faith: Mormonism is "so thoroughly American," McDermott said in a recent phone interview. "God visited [Mormon founder] Joseph Smith in upstate New York. Eden began in Missouri and the millennium will end there. The new exodus took place in North America."
None of these critiques bother Stark, who now teaches at Baylor University in Waco, Texas. He is amused by the reactions.
"The church liked the results and people who are against the church are desperate to figure out why it won't happen," he said last week. "Everyone takes the thing too seriously. I've tried to make clear all along that I was just trying to bring a little discipline to a lot of crazy conversations."
It was a game of "let's pretend," Stark says, when he applied the compound interest formula and saw huge numbers of Mormons.
He says he never meant his projections to dictate the future of Mormonism. Others may have more complex models that challenge his findings.
"They may be right," he says. "But again, if [Mormon growth] has slowed a little, it can always speed up again."
Stark, whose work will be republished this fall in a new volume, The Rise of a New World Faith: Rodney Stark on Mormonism, doesn't see any reason to apologize for his claims.
"Already there are more Mormons than Jews," he says, "and we want to consider Judaism a major world religion."