This project is a history about the decisive impact of Pelagius’ conflict with certain fifth-century Christians over salvation by faith alone. Many histories of Christianity emphasize the influence of Augustine’s conflict with Pelagius on later theologians, underscoring the latter’s role as a foil, which Luther and other Protestants rejected in favor of Augustine’s theological perspective. In this project, however, I argue that the Augustine-Pelagius conflict paled in comparison to the slightly earlier disagreement over salvation by faith alone in which Pelagius participated shortly before the Fall of Rome (ca. 410). Those faith alone debates with early fifth-century Christians whom we, following Augustine’s description, have conventionally labeled with the acronym MCSBPs, were decisive for laying out the various possibilities which later medieval and early modern Christian commentators could adopt concerning the question of salvation by faith alone. Martin Luther, like his Catholic contemporaries, encountered a back-and-forth exchange over faith alone in the pages of Pelagius’ freshly printed commentaries on the Pauline Epistles. As a result of reading the records of this heated exchange between Pelagius and the MCSBPs, Luther, like his Catholic and Protestant contemporaries, was polarized in his conclusions about salvation by faith alone. This history about Pelagius and the MCSBPs’ role in later debates over faith alone has gone unnoticed. For a variety of reasons, including Luther’s telling of his change of views on salvation, the story of Luther’s decisive shift in favor of salvation by faith alone has been placed time and again along the axes of Paul-Augustine-Luther. After reading this alternative account, however, students of Christian history will have to give careful consideration to the claim that the primary paths that led the Western Church to continual conflict over salvation by faith alone met at the intersection of Paul-Pelagius-Luther.