This thesis extends recent scholarly interest in the practical processes of Late Antique Roman law and on the integration of the episcopate into Roman power structures in the fourth century, the first century of imperial patronage of Christian communities. It confirms the "minimalist" model of Roman governance and provides a non-medieval example of the persecution of minorities as a contingent effect of competing claims to authority.This thesis argues that fourth-century elite Roman men disputing episcopal status via the Roman courts led to a transformation of episcopal polity, and that this development has been obscured by a subsequent paradigm shift in the norms concerning episcopal use of Roman law towards the end of that century.This paradigm shift identified by this thesis has three important aspects:1. With the change in imperial dynasty from the Valentinians to Theodosians, imperial favour moved from non-Nicene to Nicene bishops. Disparity of access to imperial favour during the fourth century required Nicene-identified bishops to invent tools to succeed in spite of their poor position. After the Theodosian-Nicene takeover, the Nicene-identified bishops retained these tools while also inheriting the legal framework that the non-Nicene bishops had crafted during their mid-century period of patronage.2. The power structures through which imperial favour was granted also changed. The typical fourth-century use of Roman law to resolve inter-episcopal disputes was different from that which would become established as a more enduring precedent in the Theodosian era. 3. The episcopal rhetoric used in claiming imperial favour changed from a focus on affirming one's own privilege to a focus on the proscription of others. The terminology of orthodox versus heretical is significant but must be understood as relational: even once heretics were proscribed by law, orthodoxy remained a status granted by the emperor. The methodology of this thesis argues for the importance of interpreting the relevant fourth-century sources in the context of their own time and norms, rather than in the light of the significantly different fifth-century practice as previous scholarship has done.This thesis first discusses two case studies before the paradigm shift: in Chapter One, Athanasius of Alexandria, as an example typical of the fourth century, and in Chapter Two, Priscillian of Avila, as an example at the cusp of the transition in the 380s who still demonstrates conformance with earlier practice. The thesis then describes the transition to the Theodosian-Nicene mode with an extended focus on Ambrose of Milan. Chapter Three shows Ambrose, contemporary with Priscillian, refusing to engage with existing episcopal legal practices and inventing a new strategy to survive the threat of Roman law. Chapter Four shows how Ambrose further refined this strategy in other conflicts and in doing so created a new place for bishops within the power structures of the Roman Empire.