This dissertation investigates how two fifth-century bishops of Rome, Leo I (440-461) and Gelasius (492-496) understood and opposed heresy. More specifically, by stressing the contested character of heresy and the at times optative nature of the bishop of Rome’s opposition to it, this dissertation hopes to provide a new perspective on how Leo and Gelasius imagined and justified the authority of the Apostolic See in an uncertain world. To accomplish this task, this dissertation considers Leo and Gelasius’ opposition to various different heresies and details the methods by which they were opposed. This will be done through an examination of the records of synods, Roman law, other contemporary narrative sources, but especially through the letters and tractates of Leo and Gelasius themselves, carefully read and considered in their fifth-century context. Furthermore, it is argued that the history of the development of the ideas of heresy and orthodoxy were profoundly connected with Rome’s emerging importance as a locus of authentic Christian teachings; the history of the bishops of Rome cannot be told without examining the history of heresy and orthodoxy and vice versa. Because orthodoxy and heresy were not tangible historical phenomena but rather were malleable categories that emerged as part of a wider discourse of Christian identity construction, the bishops of Rome were not in every case the unqualified enemies of heresy. Instead, their definition of heterodox belief and their opposition to religious deviance were complex, often qualified and always historically contingent. This study seeks to investigate the way in which Leo and Gelasius mobilized the language of heresiology in order to convince Christians in the Latin west and the Greek east, as well as the imperial authorities, that Rome’s interpretations were legitimate and binding.

The emperor Justinian's religious policy has sometimes been characterized as haphazard or incoherent. This dissertation examines religious policy in the Roman Empire from the accession of the emperor Justin to the inception of the Three Chapters controversy in the mid 540's AD. It considers the resolution of the Acacian Schism, Justinian's apparent ambivalence with regard to the Theopaschite formula, the attempt to court the anti-Chalcedonians in Constantinople in the period leading up to the Council of 536, and the relationship between the genesis of the Three Chapters and Second Origenist controversies. Even during these seemingly disparate episodes, this dissertation argues that it is possible to account for the apparent incoherence of this period. To do so, we create an account which includes and appreciates the embeddedness of imperial policy within a social context with two key features. First, we must bear in mind the shifting interests and information available to the individual agents through and over whom the emperor hoped to project influence. Second, we must identify the shifting and hardening symbolic and social boundaries established through the interactions of these same, competing agents. These form the basis for in- and out-group categorization. The individual interests of individual people—whether Justinian, Vitalian, Dioscorus, Leontius, Eusebius, Theodore Askidas, or Pelagius—within complex networks must always be accounted for to give a complete picture. When this social context is accounted for, Justinian's approach appears as that of a rational actor, having incomplete information, with consistent policy goals, working within inconsistent constraints to achieve those goals.