Irenaeus of Lyons (130/140-200) wrote a treatise in five books, Against Heresies, and a sort of catechism entitled Demonstratio apostolicae praedicationis.
Thanks to Adversus Haereses, the writer offers, through the study of the religious phenomenon of Gnosticism, a complete and systematic treatment of the faith. He was considered, since his contemporaries in Lyons, a shining example of orthodoxy: “We greatly esteem him for his zeal for the testament of Christ” (Eusebius, History Eccl. V, 4, 2). And this thanks also to the thorough and fine knowledge of the doctrines that he went to refute. Tertullian defines him as “a very curious explorer of all doctrines”.
Its value therefore lies both in the orthodoxy of the faith which it defends and in the fact that he is the initiator of the studies of the phenomenon of Gnosticism and one of its most ancient and authoritative sources.