Reforming efforts at the Council of Trent (1545–1563), the challenges of Protestantism, the rise of national states, and the reassessment of just war doctrine, had initiated a moment of crisis for crusading by the mid-sixteenth century. Indeed, historians have described these trends as signaling the end of the movement. This article explores the theoretical underpinnings deployed by an elite group of Spanish theologians and churchmen in May of 1567 to shore up their monarch's claim to a lucrative version of the crusade indulgence granted by popes since the fifteenth century. By rehearsing traditional arguments, eschewing those they saw as obsolete, and deploying new ones, these theorists expose the remarkable adaptability of crusading. The integrity of papally sanctioned holy war against the enemies of the faith collapsed in later centuries with the rise of international law and recognition of permanent divisions within the respublica Christiana. Yet, the ability of sixteenth-century Spanish theorists to recast ideology in the face of shifting intellectual, cultural, and social tides indicates the continuing viability of crusading during a period of inchoateness.