[5] Chosen as bishop of Lugdunum, now Lyon, his best-known work is Against Heresies, often cited as Adversus Haereses, a refutation of gnosticism, in particular that of Valentinus.

[11] Irenaeus was a Greek from Polycarp's hometown of Smyrna in Asia Minor, now İzmir, Turkey, born during the first half of the 2nd century.

The clergy of that city, many of whom were suffering imprisonment for the faith, sent him to Rome in 177 with a letter to Pope Eleutherius concerning the heresy of Montanism,[5] and that occasion bore emphatic testimony to his merits.

[12] During the religious peace which followed the persecution by Marcus Aurelius, the new bishop divided his activities between the duties of a pastor and of a missionary (as to which we have but brief data, late and not very certain).

The last action reported of him (by Eusebius, 150 years later) is that in 190 or 191, he exerted influence on Pope Victor I not to excommunicate the Christian communities of Asia Minor which persevered in the practice of the Quartodeciman celebration of Easter.

Two crania from different churches were dated by carbon-14 to the Middle Ages, but a piece of heelbone kept in the Lyon Cathedral is from the right time period.

"[25][26] But he also said, "Christ came not only for those who believed from the time of Tiberius Caesar, nor did the Father provide only for those who are now, but for absolutely all men from the beginning, who, according to their ability, feared and loved God and lived justly.

Some religious scholars have argued the findings at Nag Hammadi have shown Irenaeus's description of Gnosticism to be inaccurate and polemic in nature.

[30] Religious historian Elaine Pagels criticizes Irenaeus for describing Gnostic groups as sexual libertines, for example, when some of their own writings advocated chastity more strongly than did orthodox texts.

[32] Some of these sects were indeed libertine because they considered bodily existence meaningless; others praised chastity, and strongly prohibited any sexual activity, even within marriage.

[34][c] Eusebius attests to other works by Irenaeus, today lost, including On the Ogdoad, an untitled letter to Blastus regarding schism, On the Subject of Knowledge, On the Monarchy or How God is not the Cause of Evil, On Easter.

However, none of his works aside from Against Heresies and The Demonstration of the Apostolic Preaching survive today, perhaps because his literal hope of an earthly millennium may have made him uncongenial reading in the Greek East.

Irenaeus pointed to the public rule of faith, authoritatively articulated by the preaching of bishops and inculcated in Church practice, especially worship, as an authentic apostolic tradition by which to read Scripture truly against heresies.

[46] Irenaeus asserted that all four of the Gospels, John, Luke, Matthew, and Mark (which is the order presented in his four pillar narrative in Adversus haereses (Against Heresies) III 11,8), were canonical scripture.

[e] The success of Tatian's Diatessaron in about the same time period is "... a powerful indication that the fourfold Gospel contemporaneously sponsored by Irenaeus was not broadly, let alone universally, recognized.

[50] Scholars contend that Irenaeus quotes from 21 of the 27 New Testament books, such as:[51][f] He may refer to Hebrews 2:30 and James 4:16 and maybe even 2 Peter 5:28, but does not cite Philemon.

It was the unanimous agreement between these many independent streams of transmission that proved the orthodox faith, current in those churches, to be true.

Irenaeus repeatedly insists that God began the world and has been overseeing it ever since this creative act; everything that has happened is part of his plan for humanity.

The essence of this plan is a process of maturation: Irenaeus believes that humanity was created immature, and God intended his creatures to take a long time to grow into or assume the divine likeness.

God, however, is immortal and incorruptible, and simply by becoming united to human nature in Christ he conveys those qualities to us: they spread, as it were, like a benign infection.

[63] Part of the process of recapitulation is for Christ to go through every stage of human life, from infancy to old age, and simply by living it, sanctify it with his divinity.

In the passage of Adversus Haereses under consideration, Irenaeus is clear that after receiving baptism at the age of thirty, citing Luke 3:23, Gnostics then falsely assert that "He [Jesus] preached only one year reckoning from His baptism," and also, "On completing His thirtieth year He [Jesus] suffered, being in fact still a young man, and who had by no means attained to advanced age."

Irenaeus argues against the Gnostics by using scripture to add several years after his baptism by referencing 3 distinctly separate visits to Jerusalem.

"[65] In Demonstration (74) Irenaeus notes "For Pontius Pilate was governor of Judæa, and he had at that time resentful enmity against Herod the king of the Jews.

'"[76] The frequencies of quotations and allusions to the Pauline Epistles in Against Heresies are:[51] To counter his Gnostic opponents, Irenaeus significantly develops Paul's presentation of Christ as the Last Adam.

Irenaeus considered it important to emphasize this point because he understands the failure to recognize Christ's full humanity the bond linking the various strains of Gnosticism together, as seen in his statement that "according to the opinion of no one of the heretics was the Word of God made flesh.

[58] Irenaeus summarizes how Christ's taking human flesh saves humanity with a statement that closely resembles Romans 5:19, "For as by the disobedience of the one man who was originally moulded from virgin soil, the many were made sinners, and forfeited life; so was it necessary that, by the obedience of one man, who was originally born from a virgin, many should be justified and receive salvation.

[86][87] Irenaeus's interpretation of Paul's discussion of Christ as the New Adam is significant because it helped develop the recapitulation theory of atonement.

Irenaeus emphasizes that it is through Christ's reversal of Adam's action that humanity is saved, rather than considering the Redemption to occur in a cultic or juridical way.

[88][i] The biblical passage, "Death has been swallowed up in victory",[89] implied for Irenaeus that the Lord will surely resurrect the first human, i.e. Adam, as one of the saved.