Among characters with a claim to have changed the course of history, Martin Luther has few rivals. His own consciousness of being the harbinger of a new age could hardly have been better expressed than in his change of name: Luder became Luther, with clear echoes of Eleutherius, meaning ‘the Liberator’, and his sermons and writings quickly became an inescapable reference point for Christian communities determined to define themselves in terms sharply at odds with the established church.

The indomitable passion with which he went about his business turned Luther into the first great propagandist in the age of print. In addition to erudite theological tomes, he penned hundreds of pamphlets in stirring but accessible German. These were often illustrated with satirical woodcuts that succeeded in taking Luther's message to sectors of the population previously untouched by the written word. By one count, 183 editions of his works appeared in 1523 alone, and the responses they elicited led to a staggering tenfold rise in the output of the budding German printing presses. When combined with Luther's commitment to rendering the bible in a language intelligible to speakers of the wide range of dialects that stretched from the Low Countries to Poland, the movement served to propel the German language as the ideal cultural vehicle through which the long resentment against Italian and French domination could be expressed in close association with Luther's relentless complaints against the church of Rome.

Of course, the most important aspects of Luther's legacy are theological: his insistence on the literal meaning of scripture and his bold rejection of tradition whenever it seemed to him to contradict that meaning; his denigration of any practice, whether devotional or sacramental, that seemed like an accretion that could get in the way of an intense personal faith in Christ; his doctrine that justification comes exclusively through that faith and is unrelated to any good works; the renewed emphasis he put on the Pauline distinction between law and gospel; his related aversion to any practice or institution that claimed to mediate between the word of God and the individual believer. All these themes are central to Lyndal Roper's new biography, but the way in which she tells the story sheds new and very bright light on the piecemeal and essentially erratic development of Luther's thought. Her ‘abiding focus’, she tells us, is ‘Luther's inner development’. Where did he draw the breathtaking courage to stand up to the emperor and the most powerful princes of the empire? This, surely, was a ‘defining event’ if there ever was one. It ‘probably did more to win people over to the Reformation, and shape their hopes and expectations’, than any theological argument. But why did Luther repeatedly fall out with those with whom he had worked most closely? And how did he become the model of a married pastor after famously stating that ‘they won't wish a wife on me’? Roper charts all these emotional transformations, on the one hand, to show how Luther was radicalised by the opposition he encountered and, on the other, to clarify key moments when the arguments and attacks of others shaped the development of his own thought. There can be little doubt that it was Luther's ‘remarkable courage and sense of purpose’ that created the Reformation; and yet his ‘stubbornness and capacity to demonise his opponents nearly destroyed it’.

The result is no hagiography. What Roper has achieved is as impressive as it is rare: a deeply sympathetic yet sharply critical portrait. Melanchthon, for instance, once remembered how Luther grew doubtful about the strength of his argument and withdrew to his bedroom to pray. This points to a surprising willingness to be open to God's grace ‘as a gift he did not merit’, as Roper puts it. It was an attractive quality that endeared him to a wide and disparate range of people. In her conclusion, Roper gives three apt examples: the inimitable Albercht Dürer, who developed a vision of a global union of religions; the southern German Franciscan friar Johann Eberlin von Günzburg, who aspired to a new social order freed of the perceived sexual perversions allegedly encouraged by monastic tyranny; and the Ingolstadt lay noblewoman Argula von Grumbach, who struggled in vain for a world of intellectual equality between men and women. ‘It was Luther's genius that he could appeal to them all’, writes Roper, and ‘that they did things that they would not otherwise have dreamed of …’. But Roper never loses sight of the difficulties: Luther's writings, she tells us, ‘can be full of hatred’ and are marked by a distasteful ‘predilection for scatological rhetoric and humour’, and an ‘authoritarian, bullying, overconfident’ intransigence that was ‘more than a psychological flaw because it meant that Protestantism split very early, weakening it permanently and leading to centuries of war’. Nor was his anti‐Semitism a mere symptom of a wider contemporary prejudice. It was, Roper writes, more visceral and intrinsic to his religiosity and to his understanding of the relation between the Old and the New Testaments.

Roper does not always successfully balance her judgements. Nowhere is this clearer than in her persistent assumption that one of Luther's key innovations was an understanding of human nature that ‘escaped the split between flesh and spirit’ and rejected centuries of Catholic tradition that had condemned the sexual act and nurtured a disgust of sex as ‘polluting’. It is true that this perception can be easy to derive from the nature of the contemporary sources, dominated as they were by what Heiko Oberman once called a ‘marriage’ of Nominalism and Augustinian voluntarism in late medieval theological thought. But a brief glance at those centuries of Catholic tradition would have alerted Roper to just how misleading this perception can be. It would have allowed her to dwell more carefully on the many channels of communication that existed, but which were tragically closed by intransigence on both sides. This weighty quibble, however, should not detract from the importance of an otherwise admirably researched, elegant and accessibly written volume, one clearly destined to take its place as one of the most helpful recent contributions to our understanding of a religious genius who fascinates as often as he infuriates.