This is a learned and tightly written work which shows a panoramic mastery of medieval sources and of Luther's developing theology, situated firmly in the theological context of the later Middle Ages. Original language texts are provided in the footnotes for Saak's translations. These, with the extensive bibliography, make this a solid resource for scholars.

Four theses are developed throughout the eight chapters. Heiko Oberman's work stands as an important guide for Saak, but he does not shy away from disagreeing with his graduate teacher at points, once indicating where Oberman ‘let his own Protestant proclivities seep through, even as he relished iconoclastically smashing the idols of traditional Lutheran Luther scholarship’ (p.248).

First, Saak maintains that ‘Brother Martin’ (Luther's identification throughout the book) ‘made his discovery of passive righteousness, developed his hermeneutical and exegetical principles, and began his opposition to the papacy as the seat of diabolical incursions in the Church, as a faithful, obedient, Observant Augustinian hermit’ (p.3). Saak emphasises Luther's Augustinianism throughout (especially in chapter ii, ‘Seeking God's mercy: living the Augustinian life’, and chapter v, ‘Brother Martin Luther, Augustinian’). Luther's ‘Reformation breakthrough’ or ‘Tower experience’ (Turmerlebnis), his rupture from Rome and his developing evangelical theology, were separate occurrences, says Saak, all taking shape in the midst of Luther's life as a member of the Order of Hermits of St Augustine. Luther's intensive study of St Augustine, who became the leading influence in his theology, was in the context of the Order where, as Saak notes, ‘grace was the foundation of living the Augustinian life’ (p.82).

A second thesis is that ‘an investigation of Brother Martin Luther, Augustinian, reveals as much about the late medieval Augustinian tradition as does the late medieval Augustinian tradition about Brother Martin’ (p.3). Here Saak contributes to our understanding of later medieval Augustinian tradition through a close reading of Luther's Augustinianism in its ‘theological, religious, political, and intellectual context’ (p.4). In chapters on Luther's ‘Discoveries and breakthroughs’ and on ‘Luther's ways of Thought’, Saak thoroughly studies Luther's cognitive way of thought and his views on faith and reason. Luther accepted a ‘logic for faith’ but not the ‘logic of faith’ (p.158), making him a dialectician who could use Aristotle ‘in the realm of the external word, even in preaching about Christ and about the Trinity’. But the ‘internal Word’ is a ‘totally different realm’ for which, Luther claimed, ‘one does not become a theologian unless he does so without Aristotle’ (p.150). Logic for Luther could help to explain faith; but does not prove faith (p.151).

Third, Saak maintains that ‘Luther was a product of and contributor to the Reformation of the Later Middle Ages’ (p.4). Saak rejects a distinction between ‘late medieval reform, and early modern Reformation’. He shows Brother Martin firmly situated in the later Middle Ages context where calls to the Church had been made for ‘reform in head and members’ – a concern stretching back to the Great Western Schism (1378–1415). Luther's Ninety-Five Theses must be read in this light. The theses in themselves were not the beginning of his drive against Rome. The rupture with Rome emerged from Luther's understanding of the Church (after 1520), not his Augustinianism itself (chs vi, vii). Luther's was a ‘counter Reformation’, says Saak echoing Oberman. All earlier reformations from the twelfth century to the later Middle Ages had failed.

Fourth, Saak defends the thesis that the Lutheran Reformation failed (ch. viii). He recognises the recent work of Brad Gregory in arguing this point, ‘heartily’ ascribing to Gregory's ‘grand theory’ (p.6) and seeing his work as complementary to Gregory's. Luther himself perceived failures all around him. In the sixteenth century there was no single, common definition of ‘reformation’. There were diversities: Wittenberg, Geneva, Zurich and Münster. Attempts to ‘Christianise’ the world failed in the later Middle Ages and Luther's Reformation failed ‘if, it is seen as an attempt to Christianize significantly the flock of Christ’ (p.358). Yet Luther and Melanchthon ‘kept fighting with all their might’, believing that ‘it was only in the midst of death that we live’ (p.358).

Luther's ‘tower experience’ or Breakthrough discovery was that the just ‘live by the gift of God, namely, from faith and that this is the meaning, that the righteousness of God is revealed through the Gospel, namely, a passive righteousness, by which the merciful God justifies us through faith’ (p.118). Faith, for Luther, is Christ himself present in our heart. We perceive Christ by an intuitive cognition. Here, Luther went beyond his Ockhamist tradition, represented by Biel, who distinguished ‘evident knowledge’ from faith. For Luther, faith was more. Luther moves beyond Biel, says Saak, since, for Luther ‘faith is precisely on the most fundamental level an intuitive cognition (noticia intuitiva). Faith is a true cognition of Christ and of God's promises, and thus, in a certain way, knowledge and faith are identical for Luther, an identity that makes possible the certainty’ (pp.177–8). For Luther, Saak says, ‘faith is certain’ because ‘it is beyond us’ (extra nos; p.196). Faith is the gift of God's grace.

If so, all efforts to achieve righteousness are in vain. ‘Interior penance’ of the heart is the true penance, not the ‘external penance’ which the indulgences of Tetzel provided. ‘True penance’, writes Saak, was ‘Brother Martin's theology of the cross’ (p.277). Indulgences do not offer ‘freedom from sin and punishment’, they ‘actually increase sin by giving it license, and thereby make empty Christ's cross’ (p.279). Luther's developing ecclesiology led him to regard the pope as AntiChrist (p.342). When Exsurge Domine against Luther was burned on 10 December 1520, ‘the Rubicon had been crossed. This had been indeed Brother Martin's Reformation Breakthrough. The Reformation of the Later Middle Ages was no more. The Protestant Reformation had begun’ (pp.345–6).

Saak's stimulating study clearly shows Luther as the observant Augustinian hermit who wore his habit until October 1524. Earlier church reform efforts failed. Luther's developing theology, which led him to believe that the Gospel itself was in danger (p.298), guided him to follow his primary vow: to God.