Peter Martyr Vermigli (1499-1562) is a forgotten giant of the Protestant Reformation. With a legacy that spanned from Naples to Zurich to Oxford, Vermigli left behind him voluminous biblical commentaries and treatises, and a band of faithful disciples who collected his writings into the massive theological compendium, the Loci Communes. "On Original Sin" represents the first installment of a new project to translate the Loci into English for the first time since 1583, Presented here in a clear, readable, and learned translation, Vermigli's searching discussion of original sin reveals the biblical and patristic foundations of this controversial doctrine, and its centrality to Protestant orthodoxy. Along the way, Vermigli offers a scathing critique of the semi-Pelagian Catholic theologian Albert Pighius and defends the Augustinian understanding of sin and grace, in a treatise marked by exegetical skill, historical erudition, and philosophical sophistication.