Bishop of Lyons in Gaul (c. 130 - c. 202), the most important theologian of the second century and the first great theologian after the apostles. As a boy in Smyrna he had heard the preaching of Polycarp, who had sat under the Apostle John — giving Irenaeus a direct, two-step connection to the apostolic generation. His major surviving work, Against Heresies (five books, written c. 180), is our most important source for understanding (and refuting) the second-century Gnostic sects and the first full statement of the Rule of Faith — the doctrinal framework later codified in the Nicene Creed.
Irenaeus's achievement is a comprehensive Christian theology organized around recapitulation (anakephalaiōsis, Ephesians 1:10 — "unite all things in him"). Christ, Irenaeus argued, "went through every age" — infant, child, youth, adult — summing up and correcting the disobedience of Adam at every stage. He was the first to use "the four Gospels" as a fixed collection (though all four had been received individually before him, Irenaeus is the first to treat them collectively as the four — no more, no fewer). He gave the Church the canonical and doctrinal tools it needed to survive the Gnostic crisis: (1) apostolic succession — the bishops of churches founded by the apostles were the reliable teachers of apostolic faith; (2) the rule of faith — a short summary of essential doctrine; (3) the public Scriptures — no secret knowledge for an elite. His famous line, often quoted: Gloria Dei vivens homo — "The glory of God is man fully alive" — often taken out of context as a motivational slogan, but in Irenaeus means specifically: man fully alive is one who has come to the vision of God through the Spirit, in Christ.