Irenaeus (early 2nd century – c. AD 202) was Bishop of Lugdunum in Gaul (now Lyon, France). He was an early Church Father and apologist, and his writings were formative in the early development of Christian theology. 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. His polemical work is credited for laying out the orthodoxies of the Christian church and its faith.