We first learn of Irenaeus from a letter he carried to Eleutherius, bishop of Rome, from the churches of Viennes and Lyons, which were seeking a peaceful resolution to the Montanist controversy (Eusebius, Ecclesiastical History 5.4.2). The letter introduced Irenaeus as an esteemed presbyter in their community. By the time he returned from his embassy, we are told, a persecution of the churches in Viennes and Lyons had claimed the lives of many, including that of Pothinus, the aged bishop of Lyons. Irenaeus was installed as his successor.

We know little about the new bishop of Lyons. We do know he was not a native of Roman Gaul, where Viennes and Lyons were located, but rather from Asia Minor. Irenaeus tells of seeing and hearing in his early youth Polycarp the bishop of Smyrna (d. c. 155), a disciple of the apostle John (Against Heresies 3.3.4; Eusebius, EH 5.20.5–6). At some point he left Asia Minor for points west, likely spending time in Rome before arriving in the vicinity of Viennes and Lyons. Irenaeus’s death is usually dated to the first few years of the 3rd century. He is commemorated as a martyr, but evidence for his martyrdom is late, the earliest possible testimony coming from the pen of Jerome nearly two hundred years later.¹

Eusebius of Caesarea’s Ecclesiastical History credits Irenaeus with various treatises, but only two have come down to us: a short work entitled Proof of the Apostolic Preaching (Prf), long thought to be lost until an Armenian translation was discovered in 1904, and the work for which he is best known, A Refutation and Overthrow of Knowledge Falsely So-Called—more commonly referred to as Against Heresies (AH). The original Greek version of the five books that constitute Against Heresies was available to Armenian translators in the 6th century and was read by Photius in Baghdad in the 9th century, but has since disappeared. Of the original Greek all that remains are extracts, some lengthy, preserved in the works of other writers, and a few fragments in papyri. The fourth and fifth books of Against Heresies are preserved in Armenian in the same manuscript that holds his Proof. Fortunately, a Latin translation was made of the original Greek during the 3rd or 4th centuries. Versions of this translation have come down to us in nine Latin manuscripts, copied between the 9th and 16th centuries. Taken together, the Armenian and Latin translations preserve the work in its entirety.

It is possible that Irenaeus was the author of the Letter from the Churches of Viennes and Lyons to the Churches of Asia and Phrygia, which tells of the persecution that claimed the life of Pothinus and resulted in Irenaeus’s installation as bishop.² Eusebius quotes portions of a letter from Irenaeus to Florinus, entitled On Monarchy or That God Is Not the Author of Evil (EH 5.20.1, 4–8), and of one to Victor of Rome, in which he pleads with Victor to peacefully settle the Quartodeciman controversy with the churches of Asia (EH 5.24.11–17). The titles of other writings lost to history give some sense for the breadth of Irenaeus’s theological interests. Eusebius knows of a letter to Blastus, On Schism, a work On the Ogdoad, written when Florinus was being drawn toward Valentinian teaching (EH 5.20.1), a treatise Concerning Knowledge, written against the Greeks, and a book of discourses that contains quotations of certain passages from the Epistle to the Hebrews and the Wisdom of Solomon (EH 5.26).

We know little of Irenaeus aside from these works. But that is not to suggest we are wholly ignorant, for certain points can be divined from what has come down to us. His letter to Victor reveals that Irenaeus valued a church governance structure that preserves the communion of the churches that upheld the apostolic tradition by respecting and accepting their differences in liturgical practice, especially when those differences were rooted in tradition. Against Heresies, as well as other titles recorded by Eusebius, shows him to be a polemicist bent on defeating Valentinians, Marcionites, and others whom he regarded as deviating from the faith of the apostolic tradition. Irenaeus’s polemical argumentation, especially his descriptions of the doctrines of his opponents, quickly entered and influenced the subsequent tradition—Hippolytus, Tertullian, and Epiphanius, amongst others, all drew upon his work. But his extant works show that Irenaeus was not just a polemicist, he was also a theologian intent on preserving the tradition of the faith by promulgating his own theological account. Several features of his theology are well known, including: his identification of the Son and Holy Spirit, Word and Wisdom, as the two Hands of God (e.g., AH 4.20.1); his characterization of the divine economy as involving the growth of human beings toward perfection (e.g., AH 4.38.1; 5.6.1); his eschatology, especially his millennialism (e.g., AH 5.31–35); his affirmation of the Gospels—Matthew, Mark, Luke, and John—and their restriction to these four (e.g., AH 3.11.8–9); and his determination that the “flesh” of 1 Cor. 15:50 does not refer to the material substance of human beings but to those human beings who lack the Holy Spirit and are thus spiritually “dead” (e.g., AH 5.9–14). Less well known but no less significant to the history of Christian thought are his attribution of infinity (e.g., AH 2.1.2; 4.19.2–3) and simplicity (e.g., AH 2.13.3,8–9; 2.28.4–5) to the divine being, affirmation of the atemporal generation of the Word-Son (e.g., AH 2.13.8; 2.17.2–7), belief in the full humanity of Jesus Christ (e.g., 5.1.1; 5.14.2–3; 5.21.1), and his account of the equal divinity of the Father, Son, and Holy Spirit as the one God (e.g., AH 4.20.1–4, Prf 3–5). Irenaeus’ writings are the earliest extant record of most of these doctrines. Taken together, his polemical and constructive argumentation show he was an eclectic thinker, willing to draw upon Christian predecessors and contemporaries, Jewish traditions, ancient philosophy, and ancient literary and rhetorical theory. He ranks among the most important theologians of early Christianity and may be the most important prior to Origen.