On a hot and sticky Saturday morning this August, a massive crowd—possibly even 10,000 people—gathered in Montgomery, Alabama to protest, praise, and pray. They had bussed in en masse from as far away as California to rally in support of Alabama Chief Justice Roy Moore’s refusal to remove a nearly three ton monument of the Ten Commandments from the state judicial building. The crowd shouted “amen” as Moore vowed to fight a U.S. Circuit Court of Appeals ruling that the monument comprised an unconstitutional state endorsement of religion, and after the speech the throngs embarked on a pilgrimage down a few city blocks to view the controversial courthouse shrine. Meanwhile, a crowd of about thirty-five people according to the Associated Press’ count—hardly enough to fill a single charter bus—held a counterprotest outside of the courthouse, publicizing their belief in the constitutional commandment ordering freedom of religion. Though religious freedom appeared sidelined throughout the day’s activities, the scene in Montgomery left little doubt as to the vitality and fervency of religious practice in America.

September 11 drew attention to the prevalence and volatility of Islamic fundamentalism. But the Christian inspiration of the Montgomery rally would not comfort Jon Krakauer, who in Under the Banner of Heaven seeks to remind would-be zenophobes that religious fundamentalism has deep roots and insidious effects even within the United States. From the adrenaline-driven fanatics struggling to climb Mount Everest in Into Thin Air, Krakauer has turned his attention to a more common and more socially significant breed of zealotry. His new book centers on the story of Dan and Ron Lafferty, Mormon Fundamentalists who in 1984, claiming to have a direct revelation from God, murdered their sister-in-law and her infant daughter. This grizzly event forms the cornerstone for a book about not a single act of violence, but rather the potential peril inherent in religious belief generally, using Mormonism and the Lafferty brothers as a case study.

Under the Banner of Heaven takes on an Everest-sized scope of time, space, and genre as Krakauer traces the history of Mormonism from its formal incorporation in 1830 to the kidnapping and return of Elizabeth Smart earlier this year, and from enclaves in northern Mexico to Southern Canada to Oregon to New York. Mapping out the chronology and geography of the book could quickly rival the knotted polygamist family trees of Mormon Fundamentalists. Individual chapters read alternately as true crime page-turners, anthropological descriptions, traditional historical accounts, or shoot-em-up Westerns full of posses and wagon trains when Krakauer recounts the Mormons’ migration westward. Despite the confusion, the vast span of the book is essential to Krakauer’s argument linking the Mormons’ history of persecution and retribution to a religious basis for violence—the so-called doctrine of “blood atonement”—which influences the Lafferty brothers’ contemporary brutality and the habitual rape that accompanies the Fundamentalist Mormons’ practice of polygamy.

The epic scale of Under the Banner of Heaven shrinks dramatically when it comes to Krakauer’s perspective on religion. His preoccupation with violence leaves no room for examining the positive effects of religious faith. He never examines the practices of mainstream Mormons, members of the Church of Jesus Christ of Latter-Day Saints (the LDS Church), that have made them known not solely for their unusual beliefs, but also, by Krakauer’s own admission, for being “chaste, optimistic, outgoing, dutiful.” In the chapter about Elizabeth Smart’s kidnapping, he focuses on the excommunicated fundamentalists who forced Elizabeth to become a “plural wife” with only fleeting reference to the network of Mormons who helped the Smarts search for their daughter in the weeks after her abduction. By the last page of the book, where Krakauer explicitly states his own view on religion, it is already clear that he views all forms of faith as “capitulating to the tyranny of intransigent belief.” The book’s last official chapter closes with a statement from a former Mormon Fundamentalist turned atheist who affirms Krakauer’s fears that religion precludes thinking for yourself. In the Author’s Comments at the book’s end, however, Krakauer does introduce D. Michael Quinn, a Mormon scholar who does not believe that the church is infallible or that it has “a monopoly on truth,” yet remains faithful. Under the Banner of Heaven, like a polar opposite of the Lafferty brothers, shows little of Quinn’s balance in its critique of religion’s influence.

Krakauer entices readers with questions about the nature of religious belief, but provides no corresponding answers. To seek answers too vigorously would be to make the same mistake as the fundamentalists he writes about, whose fervent belief in a single religious truth drives them to tax fraud, polygamy, or even murder. But his questions are not too ponderous to be answered. Scholars such as William James and Harold Bloom who, among many others, provide the provocative epigraphs for each chapter (a true highlight of the book) have closely considered issues of “religious genius” and the difference between religious fanaticism and mental illness.

Krakauer’s own strict interpretation of religion and its “intransigent” beliefs clouds his ability to answer some questions. “How,” he wonders, “can a society actively promote religious faith on the one hand and condemn a man for zealously adhering to his faith on the other?” The Constitution, which guarantees religious freedom, also assumes that Americans will submit to a form of dual citizenship, practicing the religion of their choice but also adhering to the laws of the land in order to maintain the peace and freedoms of the nation. Such an idea of the divided self—part civic, part religious—dates back at least to the New Testament, in which Jesus teaches his followers to render unto Caesar that which is Caesar’s. In Krakauer’s opinion, and as proved by Dan and Ron Lafferty’s divinely inspired viciousness, faith can be the enemy of reason and the laws which reason generates, but for the founding fathers or many other religious people, faith serves as reason’s complement rather than its antithesis.

Religion dominates American culture, from crusades against terrorism to debates about abortion or gay rights, to the rally in Montgomery. Mormonism alone, already eleven million strong, is growing exponentially, and Krakauer reports projections that there will be three hundred million members of the LDS Church by the end of the twenty-first century. Given the vitality of religion in America, perhaps (to tack another question onto Krakauer’s already meaty list) the remarkable thing isn’t that the Lafferty brothers committed a heinous double murder in the name of fervent religious belief, but rather that such instances of fundamentalism gone tragically awry haven’t happened more often. Under the Banner of Heaven provides a fascinating look into the world of the LDS Church and Mormon Fundamentalism in America and a ready springboard into critical questions about the nature of religious belief and religious freedom. It’s up to the readers to take the plunge.