Irenaeus (early 2nd century-c. AD 202, also referred to as Saint Irenaeus, was Bishop of Lugdunum in Gaul, then a part of the Roman Empire (now Lyon, France). He was an early Church Father and apologist, and his writings were formative in the early development of Christian theology. He was a hearer of Polycarp, who in turn was traditionally a disciple of John the Evangelist. Irenaeus' best-known book, Adversus Haereses or Against Heresies (c. 180), is a detailed attack on Gnosticism, which was then a serious threat to the Church, and especially on the system of the Gnostic Valentinus. As one of the first great Christian theologians, he emphasized the traditional elements in the Church, especially the episcopate, Scripture, and tradition. Against the Gnostics, who said that they possessed a secret oral tradition from Jesus himself, Irenaeus maintained that the bishops.