Convinced that the antichrist was “at our gates,” the Franciscan friar, alchemist, and prophet John of Rupescissa (ca. 1310–1366) wrote The Book of the Light of the Great Magistery to help his future brethren restore a devastated Christendom after the antichrist’s inevitable downfall. Written in 1354, John’s text provides a detailed process for making the philosophers’ stone, the substance able to transmute base metals into gold and silver. With this alchemical gold and silver, the true and faithful Franciscans, the “poor men of the Gospel,” could rebuild the Church and help usher in a millennium of peace and prosperity. This volume presents the first critical edition, modern English translation, and focused study of John’s Book of the Light. Based on over one hundred surviving manuscripts, many recently discovered, this critical edition restores substantial original text that was omitted from the standard editions first printed in the sixteenth century and corrects many longstanding textual errors. In the accompanying study, Lawrence M. Principe explores John’s sources and ideas, especially his striking theories of matter and material change, and follows the transmission, reception, vernacularization, and multiple modifications of John’s text as copyists and reader-practitioners engaged with it over the next three centuries. Modern laboratory reworkings, fully illustrated and explained, go hand in hand with the textual analyses, providing both a vivid picture of the friar’s practices and observational skills and a deeper understanding of his text. John of Rupescissa stands revealed as an innovative theorist and observant practitioner; he was not an “armchair” alchemist whose knowledge came only from textual sources. The clear evidence of his alchemical practice presented here provokes, in turn, a close reevaluation of the conditions of his sixteen years of confinement at a papal prison in Avignon, where he learned and practiced much of his alchemy before writing his Book of the Light. Historians of science and technology, medievalists, book historians, manuscript scholars, and many others will find new and important information in this third volume of the Ambix series Sources of Alchemy and Chemistry.