Irenaeus of Lyons (c. 130–202) was bishop of Lyons (Lugdunum) in Roman Gaul and the most important Christian theologian of the second century. A chief opponent of Gnosticism, he defended the apostolic faith against its elaborate systems, and his work helped shape the emerging scriptural canon, the rule of faith, and the authority of the bishop's office.
A native of Asia Minor, in his youth he heard Polycarp of Smyrna, a disciple of the Apostle John. He became a presbyter of the church of Lyons and, after the martyrdom of Bishop Pothinus in the persecution under Marcus Aurelius, was chosen bishop about the year 178. In the dispute over the dating of Pascha he urged the bishop of Rome toward peace, holding that differences of observance need not break the Church's unity.
His principal work is Against Heresies, a five-book refutation of the Gnostic systems that sets out the rule of faith and the doctrine of apostolic succession; he also wrote the Demonstration of the Apostolic Preaching, a shorter exposition of Christian teaching. Both survive substantially and remain primary witnesses to the theology of the early Church.
He died about the year 202 and, by tradition, suffered martyrdom. He is venerated as a saint, and the Orthodox Church commemorates him on 23 August.
