There is some doubt as the year of Irenaeus’ birth, with estimates varying from the years 97 to 160. Most authorities settle on a year around 130. Born in Asia Minor, Irenaeus learned the Christian faith from Polycarp, the bishop of Smyrna, who was himself a disciple of the Apostle John. Irenaeus later studied at Rome and then became a presbyter in the church at Lyons, at the invitation of its first bishop, Pothinus. Lyons, then known as Lugdunum, was a flourishing trade center that soon became the most important of its kind in the West, and the principal see in Gaul. During a sudden persecution which caused the imprisonment of many of the members of the church in Lyons, Irenaeus was sent to Rome to mediate a dispute regarding Montanism, a sect of enthusiasts whose teachings Eleutherus, the bishop of Rome, seemed to embrace. On his return to Lyons around 178, Irenaeus was elected bishop, as Pothinus had been killed during the persecution.
True to his name (which means, “the peaceable one”), he acted as mediator again in a dispute in 190. Victor, the bishop of Rome, had excommunicated the Quartodecimans (the “Fourteenthers”) of Asia Minor, who celebrated Easter on the same day as the Jewish Passover, the fourteenth day of Nisan, instead of on the Sunday following the fourteenth day of Nisan, with all other Christians. Irenaeus urged patience and conciliation, and a result of his intervention, good relations were restored. Some centuries later the Quartodecimans conformed to the practice of the catholic Church of their own accord.
Irenaeus’ enduring significance rests on his writings as a theologian, in particular a large treatise entitled, The Refutation and Overthrow of Gnosis, Falsely So-Called, usually shorted to Against the Heresies. In it, Irenaeus describes the major Gnostic systems of thought, thoroughly, clearly, and often with biting sarcasm. This treatise is one of our chief sources of knowledge about second century Gnosticism. He also makes a case for teaching authority in Christianity that has deeply influenced subsequent thought, resting primarily on Scripture (of which the four Gospels are supreme) and emphasizing the interpretive authority in the continuity between the teaching of the apostles and the teaching of bishops and presbyters in the churches, generation after generation, in a visible and public succession (as opposed to the secret handing on of Gnostic doctrines from teacher to disciples). Against the Gnostics, who despised the material and exalted the spiritual, Irenaeus stressed the doctrines of the goodness of creation and of the resurrection of the body.
In his other major treatise, the Demonstration of Apostle Preaching (which was rediscovered only in 1904), he also sets out the case against Gnosticism. His principal points in this work are a clear reassertion of Christian monotheism, emphasizing the identity of the God of the Old Testament with the God of the New, and the unity of the Father and the Son in the work of revelation and redemption.
Irenaeus died at Lyons about the year 202 and was buried in the crypt of the church of Saint John (now Saint-Irenée). According to a late and uncertain tradition, he suffered martyrdom for the faith.
taken from The Oxford Dictionary of Saints
and Lesser Feasts and Fasts (1980)
Almighty God, you upheld your servant Irenaeus with strength to maintain the truth against every blast of vain doctrine: Keep us, we pray, steadfast in your true religion, that in constancy and peace we may walk in the way that leads to eternal life; through Jesus Christ our Lord, who lives and reigns with you and the Holy Spirit, one God, now and for ever. Amen.