Irenaeus ( or ; Koine Greek: Εἰρηναῖος, romanized: Eirēnaîos, IPA: [i.re̝ˈnɛ.os]) was a Greek bishop in the second century, noted for his role in guiding and expanding Christian communities in the southern regions of present-day France and, more widely, for the development of Christian theology by opposing Gnostic interpretations of Christian Scripture and defending orthodoxy. Originating from Smyrna, he had seen and heard the preaching of Polycarp, who in turn was said to have heard John the Evangelist.
Chosen as Bishop of Lugdunum, now Lyon, Irenaeus wrote his best-known work Against Heresies around 180 as a refutation of 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 said to be handed down from the apostles, and the teaching of the apostles' successors. He is the earliest surviving witness to regard all four of the now-canonical gospels as essential.
Irenaeus is venerated as a saint in the Catholic Church, Anglican Church, Lutheran Churches, the Eastern Orthodox Church, the Oriental Orthodox Churches, and the Assyrian Church of the East.
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