The history books we read as children regularly shy away from the complicated humanity of our saints and champions. After all, it’s reassuring to print the legend, often expedient to go with the two-dimensional cartoon. Only later in life, if then, do we learn that even heroes can be mean-spirited, vindictive, vulgar, envious and petty.

“Martin Luther: Renegade and Prophet,” by Lyndal Roper (Random House )

Such thoughts about image and reality are likely to trouble anyone who opens Lyndal Roper’s “Martin Luther: Renegade and Prophet.” In this new biography of the Reformation firebrand, Roper — Regius professor of history at Oxford — acknowledges Luther’s courage, intelligence and deep-seated spiritual convictions, as well as his immense cultural influence, but shows us that, by modern standards, he was anything but a model Christian. Dictatorial and uncompromising as a religious leader, a glory hog, foul-mouthed and given to scatological imagery when attacking his adversaries, disgustingly anti-Semitic and no friend of political or sexual egalitarianism, Luther comes across as, well, a fanatic. Even the endorsements on the book’s jacket second this impression: To A.N. Wilson he is “the monster genius of the Reformation” and to Hilary Mantel “a man who arouses both admiration and horror in the modern reader.”

The 19th-century historian Leopold von Ranke famously urged reconstructing the past “wie es eigentlich gewesen,” to show how it really was. Insofar as she can, Roper adapts this motto to biography: She aims to track Luther’s “inner development,” to get inside his head: “I want to know how a sixteenth-century individual perceived the world around him, and why he viewed it in this way. I want to explore his inner landscapes so as to better understand his ideas about flesh and spirit.” She adds that “it was Luther’s vivid friendships and enmities that convinced me that he had to be understood through his relationships, and not as the lone hero of the Reformation myth.” As a result, her book situates this revolutionary thinker and his thought in the sociological, political and religious crosscurrents of contemporary Germany.

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Born in 1483, Luther grew up in the mining town of Mansfeld, where his father was part of what we’d now call upper management. When young Martin toddled off to the university at Erfurt, he was supposed to come home a lawyer. But, following a terrifying epiphany during a thunderstorm, he instead resolved to become an Augustinian monk, despite paternal displeasure. Like psychologist Erik Erikson before her, Roper views Luther’s life in terms of authority figures that he initially revered, then outgrew and rejected.

As a monk, Luther adopted a lifestyle of extreme asceticism, coupled with a relentless pursuit of perfection: He was known to confess for six hours at a time. In fact, Luther seldom did anything by halves. After rising through the ranks of his monastery, the young monk was duly appointed a professor of the Bible at the small university town of Wittenberg. Roper — whose research is nothing if not detailed — tells us that there were three gates to the town, no more than nine streets and somewhere between 2,000 and 2,500 inhabitants.

On Oct. 31, 1517, Luther posted his 95 theses on the door of Wittenberg’s Castle Church. He later told disciples that his fundamental axiom — that human beings are justified by faith alone — initially came to him while sitting in the privy. In his judgment, the words of Scripture overrode both papal decrees and the teachings of the Church Fathers, the only true sacraments were baptism and Communion, and salvation depended entirely on the grace of God. Didn’t good works matter? He largely dismissed these, once likening them to the “filth” that pours out of women’s bodies, menstrual blood being, as Roper says, “the most shocking and revolting comparison he could think of.” When Luther refused to recant his increasingly heretical ideas, he was excommunicated in 1521.

The historian Lyndal Roper (John Cairns)

As Roper points out, Luther quickly grasped how modern technology — in this case, Gutenberg’s printing machine — could spread his reformist message. Between 1518 and 1525, Luther’s publications topped the European bestseller list. In most of his works, as in his private letters, he combined deft argument with vicious ridicule and rant. Bishops were sodomites; the pope was the Antichrist. Expecting martyrdom, he even boldly identified himself with Jesus. As a leader, Luther would brook no opposition, demanding of his associates, in Roper’s words, “complete intellectual and spiritual submission.” In one respect, he does strike a modern note: Against the long Catholic tradition honoring abstinence and chastity, he asserted that God wanted human beings to enjoy sex. Practicing what he preached, Luther married a former nun with whom he had several children.

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From his Scripture-based theology and the earthy vigor of his polemical prose emerged one of Luther’s finest achievements: His German translation of the Bible. Politically, he remained exceptionally conservative, abhorring social unrest and invariably supporting law and order. During the “Peasants’ War” he sided with the aristocratic establishment, proclaiming — apparently without awareness of the irony — that “nothing can be more poisonous, hurtful or devilish than a rebel.” At one point he even gave his consent to a bigamous marriage between a syphilitic landgrave and his 17-year-old concubine. Less shocking but more surprising, Luther actually published an introduction to the Koran.

Let me stress that “Martin Luther: Renegade and Prophet” isn’t written by an atheistical Christopher Hitchens wannabe, but by a highly respected historian. Roper’s tone throughout is one of evenhanded scholarly inquiry. Along the way, though, she drives home a harsh truth: People who are reasonable, empathetic and civilized make ideal neighbors but it’s usually the zealots and extremists who, for good or ill, change the world.

Michael Dirda reviews books on Thursdays in Style.