Though by 1520, the Lutheran revolt had transformed the long-standing problem of reforming the Church into a major crisis, there was no serious reform initiative from the papacy before 1536. The debate about what was wrong and what was to be done about it had however been under way from the early 1520s. An important contributor to it was the Dominican theologian Ambrosius Catharinus Politus, a former professor of civil law at the University of Siena, who in 1520 had been called upon to defend the Church against Luther, then at the height of his powers. He was not only the foremost Italian anti-Lutheran polemicist of his day, but also an ardent Immaculist and critic of the shortcomings of his own order. By 1541, he was participating in the debates over the divisive topics of predestination, justification and original sin. He attended the Council of Trent/Bologna 1545-48 as a papal theologian where he rapidly became the trusted adviser of Gian Maria del Monte, the future pope Julius III, with whose support he became first Bishop of Minori in 1547 and then Archbishop of Conza in 1551. He nevertheless continued his polemic against the Lutherans and the members of his own order. No major study of Catharinus has been written since Josef Schweizer's 'Ambrosius Catharinus Politus' in 1910. The time is now ripe for an up-to-date work on Catharinus based not only on original sources but also on a thorough acquaintance with modern scholarship on the first phase of the Counter-Reformation. This study concentrates on the last 12 years of Catharinus's life, when he emerged from relative insignificance to play an important part at the Council of Trent, and is a work of primary scholarship, involving detailed analysis of Catharinus's writings.