Irenaeus (AD 130–202) was the bishop of Lugdunum in Gaul (modern Lyons, France), a stalwart opponent of heresy, and an influential witness concerning the development of the biblical canon.
Little is known about the life of Irenaeus. We know that he was from Smyrna in Asia Minor and a student there of Polycarp (who was a disciple of the apostle John). Irenaeus moved to Rome and studied under Justin Martyr.
Sometime prior to AD 177 Irenaeus moved Lyons. In 177, Roman Emperor Marcus Aurelius ordered a violent persecution of Christians in France, but Irenaeus escaped it because at the time he was on a journey to Rome, apparently carrying a letter to the church at Rome. Irenaeus returned to France after the persecution had subsided, and he was made bishop of Lyons in 178, replacing the previous bishop, who had died or been killed in the persecution.
The significance of Irenaeus is found in his writings. There are fragments of many works, but two major works survive intact: Against Heresies and Proof of Apostolic Preaching.
Against Heresies (or The Detection and Refutation of What Is Falsely Called Knowledge, also known as Against All Heresies) is a treatise against Marcionism and Gnosticism, and especially Valentinianism, a particular form of Gnosticism that was popular in Lyons. In this treatise, Irenaeus seeks to protect his flock from heresy and to convert those who hold the Gnostic error. Until the discovery of the Nag Hammadi library in 1945, most of what was known of Gnosticism was from Irenaeus.
Irenaeus is often used to support the Roman Catholic idea of apostolic succession because he said that all the bishops can trace their succession back to the apostles. However, this statement needs to be understood in context. The Gnostics held that they were privy to secret information that had been passed down from Jesus. Irenaeus makes the case that all the orthodox bishops can trace their teaching back to the apostles, who got their teaching from Jesus. The bishops in the churches are the safeguards of the truth, which can be traced in an unbroken line back to the apostles. The line of teachers was not difficult to trace, since it had been little over 100 years since the death of the apostles. Irenaeus himself could easily trace his message to the apostle John, whose student Polycarp was Irenaeus’ teacher. The link between Irenaeus’ contemporaries and the apostles is a far cry from the more modern idea of apostolic succession.
Irenaeus may be best known for his theory of recapitulation. Recapitulation emphasizes the true humanity of Christ, who undoes the work of Adam and fulfills all that God intended for mankind. (Gnostics denied the true humanity of Christ and taught that the human body was evil.) Unfortunately, Irenaeus’ emphasis on Jesus’ human nature inherited from Mary may be seen as the basis for the later elevation of Mary as co-redemptrix, yet that was not at all what Irenaeus had in mind.
Against Heresies also speaks against Marcion, who taught that the God of the Old Testament was different from the God of the New Testament and who rejected the Old Testament. Irenaeus’ critique of Gnosticism was largely dependent upon the Old Testament, so he addressed the Marcion heresy as well.
In Against Heresies, Irenaeus also quotes from every New Testament book except 3 John, so it is an important document to show the ancient church’s acceptance of New Testament Scripture. However, Against Heresies also refers to The Shepherd of Hermas as “Scripture” and treats the book of 1 Clement as authoritative. Still, in Irenaeus we can see that the canon was beginning to coalesce.
Irenaeus’ The Proof of Apostolic Preaching (or The Demonstration of the Apostolic Preaching) is a summary of Christian preaching with an emphasis on the fact the Christ fulfills Old Testament prophecy.
Ireneaus was an influential thinker, and there is evidence that his writings were widespread in the churches around the Roman Empire during his lifetime or shortly after his death. Irenaeus died around 202. Some sources indicate that he was martyred, but we do not have enough evidence to determine the actual events surrounding his death. St. Irenaeus has been canonized by the Roman Catholic and Eastern Orthodox Churches.