This chapter takes up a famous argument by historian John Bossy that European approaches to sin changed between the medieval and early modern period from a set of social virtues and vices to a legalistic list of prohibitions. At the heart of this shift lay the introduction of the Ten Commandments as the norm for the evaluation of moral action and conscience in confession. Where medieval confessors conceptualised human action according to the seven vices, early modern confessors examined the Ten Commandments. Not least in the case of the First Commandment that is the focus of the chapter, these rules presented problems both of interpretation and application. If a Christian understanding of the First Commandment was supposed to instil the love of God, a measure of virtue ethics would be needed. The chapter shows how Catholic moral theologians struggled with the new rule-based logic in the confessional and, despite its emphasis on individual religious obedience, retained the community and social virtues as horizons for its understanding. In trying to theorise the Decalogue’s application, they produced an ever-multiplying literature of explanations. Later commentary would judge this as sterile argumentation within an ivory tower, but this is to forget its initial impetus, which was to train theologians to examine and correct sin. The confrontation of the Ten Commandments with confessional practice thus helped foster the early modern casuistry that has become a quintessential case of ethical rule density, but also reminds us of the inextricable nature of rules and virtue.