Irenaeus is thought to have been a Greek from Polycarp's hometown of Smyrna in Asia Minor, now Izmir, Turkey. He was brought up in a Christian family, rather than converting as an adult, and this may help explain his strong sense of orthodoxy. Irenaeus was one of the first Christian writers to use the principle of apostolic succession to refute his opponents.
Irenaeus is remembered as the second bishop of Lyons, although there is no clear evidence that he ever officially assumed the episcopal duties. The first bishop, Pothinus, was martyred around 177 during persecutions under Marcus Aurelius, when Irenaeus was visiting Rome.
Irenaeus is remembered as a martyr, although there is no evidence for how he died, presumably shortly after the turn of the third century. He was buried under the church of Saint John's in Lyons, which was later renamed St. Irenaeus. His tomb and his remains were destroyed in 1562 by the Calvinist Huguenots. The remains of Leonardo da Vinci and Kepler, among others, also were lost in the religious wars of those times.
Irenaeus wrote a number of books, but the most important that survives is the five-volume On the Detection and Overthrow of the So-Called Gnosis, normally referred to as Adversus Haereses (English: Against Heresies). Only fragments in its original Greek exist, but a complete copy exists in a wooden Latin translation, made shortly after its publication in Greek, and Books IV and V are present in a literal Armenian translation.
The purpose of Against Heresies is to refute the teachings of various gnostic groups. Until the discovery of the Library of Nag Hammadi in 1945, Against Heresies was the best surviving description of Gnosticism.
Irenaeus cites from most of New Testament canon, as well as the noncanonical works 1 Clement and The Shepherd of Hermas, however he makes no references to Philemon, 2 Peter, 3 John and Jude. Irenaeus was the first Christian writer to list all four and exactly four of the now canonical Gospels as divinely inspired, possibly in reaction to Marcion's edited version of Gospel of Luke, which he asserted was the one and only true gospel.
The central point of Irenaeus' theology is the unity of God, in opposition to the Gnostics' division of God into a number of divine "Aeons", and their distinction between the "High God" and the wicked "Demiurge" who created the world. Irenaeus uses the Logos theology he inherited from Justin Martyr, but prefers to speak of the Son and the Spirit as the "hands of God". Christ, for him, is the invisible Father made visible.
His emphasis on the unity of God is reflected in his corresponding emphasis on the unity of salvation history. Irenaeus repeatedly insists that God created the world and has been overseeing it ever since. Everything that has happened is part of his plan for humanity. The essence of this plan is maturation: Irenaeus believes that humanity was created immature, and God intended his creatures to take a long time to grow into his likeness. Thus, Adam and Eve were created as children. Their Fall was thus not a full-blown rebellion but a childish spat, a desire to grow up before their time and have everything now.
Everything that has happened since has therefore been planned by God to help humanity overcome this and grow up. The world has been designed by God as a difficult place, where human beings are forced to make moral decision - only in this way can they mature. Irenaeus likens death to the whale that swallowed Jonah: it was only in the depths of the whale's belly that Jonah could turn to God and do his will. Similarly, death and suffering appear evil, but without them we could never come to know God.
The high point in salvation history is Christ. Irenaeus believes that Christ would always have been sent, even if humanity had never sinned; but the fact that they did sin determines his role as a saviour. He sees Christ as the new Adam, who systematically undoes what Adam did: thus, where Adam was disobedient about the fruit of a tree, Christ was obedient even to death on the wood of a tree. Irenaeus is the first to draw comparisons between Eve and Mary, contrasting the faithlessness of the former with the faithfulness of the latter. In addition to reversing the wrongs done by Adam, Irenaeus thinks of Christ as "recapitulating" or "summing up" human life. This means that Christ goes through every stage of human life, from infancy to old age, and simply by living it, sanctifies it with his divinity. Irenaeus is therefore forced to argue that Christ did not die until he was quite old!
Irenaeus thus thinks that our salvation comes about, essentially, through the incarnation of God as man. He characterises the penalty for sin as death and corruption. God, however, is immortal and incorruptible, and simply by becoming united to human nature in Christ he conveys those qualities to us: they spread, as it were, like a benign infection. Irenaeus therefore understands the atonement of Christ as happening through his incarnation rather than his crucifixion, although the latter is an integral part of the former.