This dissertation is a historical study of Gennadius of Marseilles’ De viris illustribus, a catalog of 101 short, bio-bibliographical notices devoted to Christian authors and their works written by a member of the ascetic movement in Marseilles during the second half of the fifth century A.D. The study advances existing knowledge of the fifth century in three distinct ways. First, it establishes the Sitz im Leben of De viris illustribus. To do so, it has had to contend with not only a dearth of evidence relating to the life and literary practices of its author but also the longstanding but nevertheless erroneous assumption that in writing De viris illustribus Gennadius produced nothing more than a second part of what Jerome had written nearly eight decades earlier. Second, on the basis of the established context, it uses two case studies to explain how Gennadius presented three western theological controversies—Origenism, Pelagianism, and “Semipelagianism”—that took place during the eight decades that separated him from his literary predecessor. And third, it uses De viris illustribus, both its historical and theological contexts and its content, to reevaluate the character of southern Gallic asceticism during the second half of the fifth century, especially its close engagement with several eastern or eastern-oriented texts, most notably those of Origen of Alexandria, Evagrius Ponticus, and Rufinus of Aquileia. The effect of this reevaluation not only sheds light on the theological substrate that animated Massilianism but also explains its opposition to Augustine of Hippo’s late theology in what has been known since the sixteenth century as the Semipelagian Controversy.