The magisterial Adversus Haereses by the second-century apologist and bishop Irenaeus of Lyons takes on the daunting task of refuting the heresies that had begun springing up in the Christian world. Book 2 of this five-book work refutes the gnostic and Marcionite heresies—the “falsely so-called knowledge”—that Irenaeus so thoroughly exposed in Book I, convincingly and effectively stating the clear line between orthodox belief and gnostic doctrine. This new translation and introduction, based on the most up-to-date sources, is an invaluable addition to scholarship on the most significant work by one of the most renowned authors in early Christianity. †