This essay argues that the effort to define and police the shifting boundaries of Christianity in early modern Spain produced a counter-intuitive peninsular logic of secularization. Hoping to stem the extension of ecclesiastical power over everyday cultural life, New Christians and religious reformers defended the authority of civic tribunals to regulate the social conditions of faith. This attempt to secularize the crime of heresy was not simply a call for the Crown to execute a coercive religious agenda. On the contrary, by turning late sixteenth- and early seventeenth-century inquisitorial regimes of discipline and dissimulation to critical pastoral and political ends, New Christians such as the negotiator Francisco Núñez Muley and reformers such as the court historian Pedro de Valencia sought to transform both Christian orthodoxy and civic identity. This social and intellectual history troubles conventional scholarship on secularization, which tends to focus on the legacy of the Protestant Reformation and Northern European Enlightenment while either lamenting an apparent theological backwardness south of the Pyrenees or ignoring the peninsula altogether. It is not my goal to trace an alternative genealogy of late modern secularism to the various coercive discourses and practices associated with early modern inquisition. I argue rather that the candidness with which Spanish intellectuals justified the pastoral and disciplinary features of civic law can serve as a guide for untangling the complex relationship between secularism and religion in other times and places.