He was probably born about the year 125, in one of those maritime provinces of Asia Minor where the memory of the apostles was still cherished and where Christians were numerous. He was most influenced by St. Polycarp who had known the apostles or their immediate disciples
Many Asian priests and missionaries brought the gospel to the pagan Gauls and founded a local church. To this church of Lyon, Irenaeus came to serve as a priest under its first bishop, St. Pothinus, an oriental like himself. In the year 177, Irenaeus was sent to Rome. This mission explains how it was that he was not called upon to share in the martyrdom of St Pothinus during the terrible persecution in Lyons. When he returned to Lyons it was to occupy the vacant bishopric. By this time, the persecution was over. It was the spread of gnosticism in Gaul, and the ravages it was making among the Christians of his diocese, that inspired him to undertake the task of exposing its errors. He produced a treatise in five books in which he sets forth fully the inner doctrines of the various sects, and afterwards contrasts them with the teaching of the Apostles and the text of the Holy Scripture. His work, written in Greek but quickly translated to Latin, was widely circulated and succeeded in dealing a death-blow to gnosticism. At any rate, from that time onwards, it ceased to offer a serious menace to the Catholic faith.