A second-generation Lutheran theologian and church leader, Matthias Flacius Illyricus (1520–75) is remembered for both his keen mind and his sharp polemics. In this study of Flacius' life and work, Luka Ilić examines why he became such a polarising figure. A native of Croatia, Flacius embraced Protestantism as a young man, with the result that he lived most of his life as a foreigner in evangelical lands far from the Adriatic. Upon arriving in Wittenberg in 1541, Flacius studied under Martin Luther and Philip Melanchthon. Though he remained devoted to Luther, Flacius became a bitter critic of Melanchthon's work under the Augsburg Interim of 1548. Ilić shows how conflicts begun in this period set the tone for Flacius' career, as he continued to engage in controversies with theologians around Melanchthon, including Georg Major, Johannes Pfeffinger and Viktorin Strigel. Ilić then identifies the conflicts with Strigel (a colleague in Jena) as the start of a second period of radicalisation, in which Flacius' interpretation of original sin eventually cost him influential posts as a teacher and church official. Despite lasting contributions to Lutheran scholarship like The Magdeburg centuries and Clavis Scripturae, Flacius spent the rest of his life in exile, with new controversies growing up around him and old animosities following him. In this regard, the ‘radicalisation’ thesis that Ilić puts forth proves its worth: it allows Flacius to stand as a prominent theologian in his own right, while recognising the extent to which he was a truly divisive figure. Ilić sums up the effect of Flacius' radical theology as follows: ‘The lack of willingness to compromise that came to characterize Flacius ever more strongly from his Magdeburg years onward meant that he irreversibly damaged his reputation and as a result partially destroyed the very network he had been building up so carefully over his life’ (p. 228). Despite this tainted legacy, Flacius has remained a compelling figure and thinker of the Reformation. Strengths of this study include thorough biographical research (probably the best currently available in English) and the inclusion of many original sources. Readers unfamiliar with controversies surrounding Flacius might want to supplement this book with sources that describe the perspectives of his opponents, since these receive little mention here. In light of the book's title, Theologian of sin and grace, one learns more about Flacius' doctrine of sin than about his concept of grace. Of course, this heavy emphasis on sin and less overt attention to grace stems from Flacius himself and further reminds readers why he inspired such strong reactions in his lifetime and beyond. This book makes valuable contributions to the study of Matthias Flacius Illyricus and the confessional developments of the sixteenth century.