This dissertation argues that a particular—and often overlooked—strand of natural law theory played an essential role in arguments for the secularization of political power in the sixteenth and seventeenth centuries. Between the start of the Protestant Reformation (1517) and the English Restoration (1660), European conceptions of political and legal authority underwent a series of sweeping changes. Among the most drastic of these changes was the secularization of the idea of civil authority, which consisted of three developments. First, the legitimacy of civil sovereigns was no longer dependent on religious qualifications. Heretics and pagans could hold legitimate civil authority over Christian subjects. Second, civil authority came to be seen as the product of human agency rather than divine will alone. Kings were placed on their thrones by their subjects and were thus accountable to the communities they governed. Third, civil jurisdiction was limited to the pursuit and enforcement of temporal goods: civil peace, personal security, and the public virtues necessary for these ends. Civil sovereigns no longer had the right to determine citizens’ religious, spiritual, or supernatural obligations. My dissertation demonstrates that these three developments were made possible by the philosophical framework of natural law, which was deployed by both Catholic and Protestant political thinkers of this period.