Irenaeus of Lyons (fl. 180) was a second-century bishop whose work shaped the Scriptures, exegesis, institutions, theology, and spirituality of early Christianity so profoundly that his imprint remains discernible almost two millennia later. Through his teacher, Polycarp, Irenaeus retained a link to the first Christian generation, since Polycarp had talked “with John and with others who had seen the Lord” (Eusebius Hist. eccl. 5.20.6). Eusebius preserved part of the letter in which Irenaeus described his early years with Polycarp in Smyrna (modern-day Tu…