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 basic aim of the thesis is to investigate whether the Romans of the East (Byzantines) during the 6th to the 8th centuries were interested in the Latin west and the imperial rule over it. In the first part of the work, the various discourses concerning the origins of the Justinianic conquest or reconquest of Africa and Italy articulated in the contemporary sources are identified and evaluated; the only one which cannot be shown or supposed to have been officially articulated is the discourse of a source of Pseudo-Zachariah Scholasticus which makes African and maybe also Italian exulants complaining in the imperial court about the local rulers responsible for the Vandal and Gothic wars. After some other preliminary studies (e.g. concerning the so-called problem of Theodericʼs constitutional position), it is concluded that the emperor Justinian was not interested in an ideologically founded restoration of the empire, but made the western wars because of his contacts with western aristocrats. In the second part of the thesis, the presentation of the Justinianic western wars and western events or realities in the works of the Byzantine historians from Marcellinus Comes and Procopius to Theophylactus Simocatta (the 6th to the early 7th centuries) is investigated and an attempt is made to explore...