City Readers Digital Historic Collections at the New York Society Library

Irenaeus (; Greek: Εἰρηναῖος Eirēnaios; c. 130 – c. 202 AD) was a Greek cleric noted for his role in guiding and expanding Christian communities in what is now the south of France and, more widely, for the development of Christian theology by combatting heresy and defining orthodoxy. Originating from Smyrna, now Izmir in Turkey, he had heard the preaching of Polycarp, who in turn was said to have heard John the Evangelist.

Chosen as bishop of Lugdunum, now Lyon, his best-known work is On the Detection and Overthrow of the So-Called Gnosis, often cited as Adversus Haereses, an attack on gnosticism, in particular that of Valentinus. To counter the doctrines of the gnostic sects claiming secret wisdom, he offered three pillars of orthodoxy: the scriptures, the tradition handed down from the apostles, and the teaching of the apostles' successors. Intrinsic to his writing is that the surest source of Christian guidance is the church of Rome, and he is the earliest surviving witness to regard all four of the now-canonical gospels as essential.

He is recognized as a saint in the Catholic Church on 28 June, and in the Eastern Orthodox Church on 23 August.