Irenaeus of Lyon ( 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. He was declared a Doctor of the Church in the Catholic Church by Pope Francis in 2022.
Irenaeus was a Greek from Polycarp's hometown of Smyrna in Anatolia, now İzmir, Turkey, born some time before Polycarp's death circa 155 AD. Unlike many of his contemporaries, he was brought up in a Christian family rather than converting as an adult.
During a local persecution of Christians around Lyon during the reign of Marcus Aurelius, the Roman emperor from 161 to 180, Irenaeus was a priest of the Church of Lyon. 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, and that occasion bore emphatic testimony to his merits. While Irenaeus was in Rome, the Lyon persecution took place. Returning to Gaul, Irenaeus succeeded the martyr Saint Pothinus and became the second bishop of Lyon.
During the religious peace which followed the Lyon persecution, 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). Almost all his writings were directed against Gnosticism. The most famous of these writings is On the Detection and Overthrow of the So-Called Gnosis, usually known by the abbreviated title Against Heresies (Adversus haereses). Irenaeus alludes to coming across Gnostic writings, and holding conversations with Gnostics, and this may have taken place in Anatolia or in Rome. However, it also appears that Gnosticism was present near Lyon: he writes that there were followers of 'Marcus the Magician' living and teaching in the Rhone valley.
Little is known about the career of Irenaeus after he became bishop. 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 Anatolia which persevered in the practice of the Quartodeciman celebration of Easter.
Nothing is known of the date of his death, which must have occurred at the end of the second or the beginning of the third century. He is regarded as a martyr by the Catholic Church and by some within the Orthodox Church. He was buried under the Church of Saint John in Lyon, which was later renamed St Irenaeus in his honour. The church was devastated in 1562 by the Huguenots. Several relics supposedly of Irenaeus are held in various churches in Lyon. 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.
The Latin Catholic Church celebrates Irenaeus' memorial on 28 June. Pope Francis declared Irenaeus the 37th Doctor of the Church on 21 January 2022. Francis also conferred upon Irenaeus the supplementary title Doctor unitatis ("Doctor of Unity").
The Eastern Orthodox Church celebrates Irenaeus, the feast being on 23 August.
Lutheran Churches honor Irenaeus on their calendar of saints on 28 June.
Irenaeus is honored in the Church of England and in the Episcopal Church on 28 June.
Irenaeus wrote a number of books, but the most important that survives is the Against Heresies (or, in its Latin title, Adversus haereses), written around 180. In Book I, Irenaeus talks about the Valentinian Gnostics and their predecessors, who he says go as far back as the magician Simon Magus. In Book II he attempts to provide proof that Valentinianism contains no merit in terms of its doctrines. In Book III, Irenaeus attempts to show that these doctrines are false, by providing counter-evidence gleaned from the Gospels. Book IV consists of Jesus's sayings, and here Irenaeus also stresses the unity of the Old Testament and the Gospel. In the final volume, Book V, Irenaeus focuses on more sayings of Jesus plus the letters of Paul the Apostle.
Irenaeus wrote: "One should not seek among others the truth that can be easily gotten from the Church. For in her, as in a rich treasury, the apostles have placed all that pertains to truth, so that everyone can drink this beverage of life. She is the door of life." 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. . . and desired to see Christ and to hear His voice."
The purpose of "Against Heresies" was to refute the teachings of various Gnostic groups; apparently, several Greek merchants had begun an oratorial campaign in Irenaeus's bishopric, teaching that the material world was the accidental creation of an evil god, from which we are to escape by the pursuit of gnosis. Irenaeus argued that the true gnosis is in fact knowledge of Christ, which redeems rather than escapes from bodily existence.
Until the discovery of the Library of Nag Hammadi in 1945, Against Heresies was the best-surviving description of Gnosticism. Some religious scholars have argued the findings at Nag Hammadi have shown Irenaeus's description of Gnosticism to be inaccurate and polemic in nature. However, the general consensus among modern scholars is that Irenaeus was fairly accurate in his transmission of gnostic beliefs, and that the Nag Hammadi texts have raised no substantial challenges to the overall accuracy of Irenaeus's information. 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. However, the Nag Hammadi texts do not present a single, coherent picture of any unified gnostic system of belief, but rather divergent beliefs of multiple Gnostic sects. 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.
Irenaeus also wrote The Demonstration of the Apostolic Preaching (also known as Proof of the Apostolic Preaching), an Armenian copy of which was discovered in 1904. This work seems to have been an instruction for recent Christian converts.
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.