The Augustan laws criminalising adultery and stuprum and promoting marriage and childrearing not only intruded into the family lives of citizens (including freedpersons and their descendants) but also made marital probity central to moral and political discourse in the first century. This was true not only for imperial figures like Seneca and Musonius Rufus, but also for Jews and the earliest Christians. Considering Philo and Paul as interpreters of the sixth commandment (‘you shall not commit adultery’) illuminates the subtle but significant ways that the Roman matrix set the parameters within which they worked out their arguments. For Philo the ten commandments are heads or summaries of the legislation as whole; the sixth commandment (following the LXX) takes pride of place in the ‘second pentad’ because adultery is the greatest of injustices and is rooted in pleasure, the most fatal of passions. He reads the commandment expansively and through first-century constructions of sexuality. Comparison with Pseudo-Phocylides suggests that Philo did not originate these positions, but shares them with other first-century interpreters. Paul also is concerned with summarising the law; he cites the sixth commandment in Romans, where he grants it first place in the second pentad, and reads it as prohibiting all unions and acts that contravened good sexual mores. But for Paul, the sixth commandment is no longer a guide for the blind; it is never cited when he advises his communities on sexual morality. Instead it supports his argument for freedom from the law in Christ.