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Like many devout people—our president, for one—brothers Ron and Dan Lafferty answer to a divine power. Mormon fundamentalists, the Laffertys paid a visit to a third brother’s home in American Fork, Utah, on a sunny day in July 1984. There they killed their brother’s wife and her infant daughter with a butcher knife. Why? Because God had told them to. And God, their god anyway, doesn’t take kindly to having His orders disobeyed.

The Laffertys are—let’s not mince words here—religious fanatics. And fanaticism is a subject Jon Krakauer knows a thing or two about. His first book, Into the Wild, focused on a young man’s ill-fated attempt to survive alone in the wilds of Alaska. Into Thin Air, the book that made Krakauer famous, was a firsthand account of disaster on Mount Everest, the Olympus of extremism. In Under the Banner of Heaven: A Story of Violent Faith, Krakauer uses the Laffertys to demonstrate that “when religious fanaticism supplants ratiocination, all bets are suddenly off. Anything can happen. Absolutely anything. Common sense is no match for the voice of God…”

But Krakauer teases out another theme in this well-researched and well-written book; namely, that in order to understand the Lafferty brothers’ ability to kill without remorse or compunction, one has to know something about what author (and son of a Mormon) Mikal Gilmore once described as Mormonism’s “history of astonishing violence.” For if the 21st-century face of the church is one of hard working, all-American normality—blond ambition in ritual undergarments—the early years of the Church of Jesus of the Latter-Day Saints were marked by a kind of psychopathic weirdness rarely encountered outside the Old Testament. Without some knowledge of Mormonism’s bloody past—which the church has gone to great lengths to whitewash—there is no comprehending the Laffertys’ brutal crimes.

The brothers, like the 30,000 to 100,000 other fundamentalist Mormons estimated to be living in North America today, embrace beliefs and practices that the mainstream Mormon church had renounced—albeit only under duress—to win itself a place at the table of American democracy. These include polygamy (or, as the “Saints” call it, plural or “celestial” marriage); a belief in the inherent inferiority of blacks; and the practice of blood atonement—the ritual spilling of sinners’ blood.

Krakauer dedicates several long chapters to exploring the religion’s past. He describes founder Joseph Smith’s 1827 “discovery”—through the intercession of the angel Moroni—of several hieroglyph-covered golden plates, which Smith “translated” using a pair of magic spectacles into the Book of Mormon, that hodgepodge of historical absurdities and existing theology that Mark Twain once dismissed as “chloroform in print.” Krakauer also chronicles the sect’s wanderings westward—marked by persecution by non-Mormons and much bloodletting by the Mormons themselves—to find the place where the Lord wanted Smith to establish a theocracy. The journey led from New York to Ohio to a site near present-day Independence, Mo.—which Smith, in what has to be one of the least convincing revelations in the long and checkered history of revelations, declared to be the original site of the Garden of Eden—to Illinois and finally to Utah, a spot so miserable the wanderers were certain no one would hassle them there.

By the time they got to Utah, of course, Smith was dead—killed by a mob incited by his destruction of the offices and press of a newspaper that had dared print the truth about his secret harem of wives—and Brigham Young, the president of the church’s Quorum of the Twelve Apostles, had taken over as “prophet.” And whereas Smith—who once famously threatened to establish Mormonism “by the sword,” making “one gore of blood from the Rocky Mountains to the Atlantic Ocean”—was largely a talker, Young was a doer. Krakauer describes Young’s reaction in 1861 upon seeing a crude cross marking the site of the Mountain Meadows Massacre. It was there, in September 1857, that a Mormon militia ruthlessly slaughtered some 140 “gentiles”—non-Mormon men, women, and children alike—in an episode that exemplified “the fanaticism and concomitant brutality of a culture that would be so enthusiastically idealized a century later by Dan Lafferty and his fundamentalist brethren.” Studying the cross, which bore the words, “Vengeance is mine…saith the Lord,” Young asserted, “Vengeance is mine, and I have taken a little.” He then ordered the cross torn down.

According to Krakauer, the fundamentalists believe the church lost its way in 1890, when Wilford Woodruff, Mormonism’s fourth prophet, under increasing government pressure to start abiding by U.S. laws or else, announced that God had revealed to him the necessity of ending the sacred practice of plural marriage. This was quite a turnabout from only four years earlier, when the Lord informed then-Prophet John Taylor, “I have not revoked this law [of polygamy] nor will I for it is everlasting and those who will enter into my glory must obey the conditions thereof, even so Amen.” The Mormon god, it seems, had discovered pragmatism.

To fundamentalists, Woodruff’s about-face reeked of political expediency, as did Spencer W. Kimball’s 1978 decree allowing black males into the Mormon priesthood. Periodically since then, Mormons dissatisfied with such reversals have broken from the church and relocated to out-of-the-way places where they could follow the old practices free of government interference. They haven’t completely managed to avoid publicity; as demonstrated by theLeBaron clan murders and the prosecution of a fellow named Tom Green for felony child rape following his marriage to 13-year-old Linda Kunz—he also happened to be married to her mother—no place is far enough out there if you’re far enough out there.

Krakauer takes us to such places as Colorado City, Ariz., and Bountiful, British Columbia, and the picture he paints is not a pretty one. In Colorado City, the faithful are “forbidden to watch television or read magazines or newspapers”—a state of affairs that leads Krakauer to note, dryly, “[L]ife in Colorado City…bears more than a passing resemblance to life in Kabul under the Taliban.” But fundamentalism’s biggest victims are young girls, who are regularly forced—by threat of damnation or much worse—to become the plural wives of men many years their senior. The practice, he notes, amounts to little more than pedophilia and rape, and the girls have little recourse in the legal system, for in places like these the police, like everyone else, answer to the Lord, not the law.

It was to this milieu that the Lafferty brothers gravitated after leaving the mainstream Mormon church. Krakauer does an excellent job of chronicling their metamorphosis from law-abiding citizens to fanatical polygamists who thought nothing of beating their wives for minor infractions—women are little more than chattel in Mormon fundamentalist circles—or of breaking any law they decided was in conflict with God’s Law.

Still, if most fundamentalist Mormon men live narrow, repugnant lives, they seldom commit homicide. So what led the Lafferty brothers to kill a woman and her young baby—the wife and child of their brother, no less? The answer, says Krakauer, lies in yet another discredited—by the mainstream church, anyway—Mormon practice: that of looking to God for guidance in the form of divine revelation. As Krakauer notes, Mormonism began as a religion in which everyone was encouraged to go directly to God for direction. But Smith quickly recognized that his power rested in his status as the sole recipient of the word of God. He tried to amend the theology through—what else?—yet another revelation, but, as happened with polygamy and institutionalized racism, its memory lingered. The Laffertys wound up members of a sect led by Robert Crossfield, who went by the name Onias and founded the School of the Prophets near Provo, Utah, to teach his followers (all male, naturally) the sacred art of receiving divine revelations.

Now, the problem with revelations is that they often bear all the hallmarks of wish fulfillment. In short, God tells you exactly what you want to hear. One of Smith’s first “divine” revelations, for instance, directed one Martin Harris to pay Smith’s $3,000 printing bill for the Book of Mormon. Lest Harris dither, God threatened the destruction of Harris and his property. Harris paid. In another revelation, God let it be known that Smith’s wife, Emma, who had let her husband know exactly what she thought about his notions on celestial marriage, was to get with the polygamy program or else. Emma said no way, and God, who evidently knew when to shut up, stopped bothering her. As for Smith, he simply refrained from discussing his other marriages with Emma.

But it’s one thing for the Lord to order somebody else to pay your bills, another for Him to order the murder of innocents. Yet these were the Lord’s instructions to Ron Lafferty. Ron’s fundamentalism had already cost him dearly. His wife had left him, taking their children; his businesses had failed; he was living in his car. So when he received God’s orders to “remove” the very individuals whom he held responsible for the breakup of his marriage, it must have been with a certain amount of satisfaction. At the top of the list—one of the most frightening aspects of the Lafferty case was that they had further victims in mind—was his brother’s wife and her child. (The former had helped convince Ron Lafferty’s wife to leave him. As for the latter, Ron dismissed her as “a child of perdition.” Besides, he reasoned, killing her would be a “blessing,” because she would otherwise be deprived of her mother.) In another revelation, God directed Dan Lafferty to perform the actual “removals.” This Dan did, slitting the throats of Brenda Lafferty, 24, and her 15-month-old child, Erica Lane Lafferty. As he approached the latter, he says he told her, “I’m not sure what this is all about, but apparently it’s God’s will that you leave this world; perhaps we can talk about it later.” Then he virtually decapitated her with the butcher knife.

It’s relatively easy to write off the Laffertys as a couple of berserker nutballs. But as Krakauer makes clear—and the testimony of the prosecution team’s psychologists at Ron Lafferty’s murder trial made obvious—their dependence on and adherence to directions from above was not only “sane,” but in keeping with the beliefs of many religious sects worldwide. The Lafferty boys were only doing what they were taught—checking in with God and faithfully following out his instructions. Krakauer quotes Will Bagley, author of a provocative study of Mormonism called The Blood of the Prophets, who notes that “Brigham Young’s relentless commitment to the Kingdom of God forged a culture of violence…that bequeathed a vexatious heritage to his successors.” As the Lafferty brothers illustrate, that heritage is alive and flourishing. CP