The sixteenth-century Spanish Empire encompassed a wide array of ethnic peoples, with differing legal identities that provided them distinct avenues to political enfranchisement. Did any recognizable set of standards govern who was eligible to become a vecino (urban citizen) in Spanish cities, or who was able to present him or herself in front of a court of law as an "Old Christian"? How were "Indios," "Moriscos," and their descendants accommodated within Spanish society? Through a detailed study of legal, municipal, and ecclesiastical sources, this dissertation challenges existing arguments that highlight the importance of limpieza de sangre ("purity of blood") in determining the boundaries of inclusiveness. Instead, it argues that the reforms of the Council of Trent (1545-1563) provided the central tenets for the monarchy's citizenship discourse.In 1565, the Spanish Crown adopted legislation from the Council of Trent as royal law. A generation later, the rhetoric employed in religious reform councils andcatechetical manuals found its way into the legal depositions of the empire's multi-ethnic subjects. After the 1560s, membership in Castilian society's institutions became contingent upon the demonstration of policía cristiana (Christian citizenship).This manuscript illustrates how Christianity and policía became central to the common identity of members of the Spanish kingdoms, while re-examining questions of race, ethnicity, and belonging in Spain and Latin America. It does so by analyzing the political transformation from the viewpoints of royal agents, colonial settlers, and indigenous inhabitants of the Crown's peripheral territories. While royal decrees and ecclesiastical memoranda illuminate the motives and concerns of the elite, legal trials expose the mentalités of more humble sections of society, as well as the cultural practice of the law. Additionally, an analysis of two popular religious devotions demonstrates how subalterns were able to amend the Christian citizenship discourse by altering local definitions of Christianity through symbolic imagery. In the end, this study reveals conflict that arose between two recognizable political factions within the empire: those who envisioned a multi-ethnic polity united by a common Christian identity and "Old Christian" nativists who sought to protect the traditional ethnic, cultural, and legal divisions of religiously-plural medieval Spain.