The writings of Irenaeus give him an honored place among the Fathers of the Church for they laid the foundations of Christian theology and, by refuting the errors of the Gnostics, kept the youthful Catholic faith from the danger of corruption by the subtle, pessimistic doctrines of these philosophers. Irenaeus was born, probably about the year 125, in one of the maritime provinces of Asia Minor, where the memory of the Apostles was still cherished and where Christians were already numerous. His education was exceptionally liberal, for, besides a thorough knowledge of the Scriptures, he had an acquaintance with Greek philosophy and literature. Irenaeus had also the privilege of sitting at the feet of men who had known the Apostles. Of these the one who made the deepest impression on him was St. Polycarp, the venerable bishop of Smyrna. All through his life, he told a friend, he could recall every detail of Polycarp's appearance, his voice, and the very words he used when telling what he had heard from John the Evangelist and others who had seen Jesus.
From early times commerce had been brisk between the ports of Asia Minor and the city of Marseilles, at the mouth of the Rhone River. In the second century of the Christian era Levantine traders were conveying their wares up the river as far as Lyons, the most populous city of Gaul and an important mart for all Western Europe. In the train of these Asiatic merchants, many of whom settled in Lyons, came Christian missionaries, who brought the Gospel to the pagan Gauls and founded a vigorous church. Here Irenaeus was sent to serve as priest under the bishop, Pothinus.
The high regard which Irenaeus earned for himself at Lyons was shown in the year 177, when he was chosen to go on a serious mission to Rome. He was the bearer of a letter to Pope Eleutherius, urging him to deal firmly with the Montanist faction in faraway Phrygia, for heresy was now rampant in the East. This mission explains how it was that Irenaeus did not share in the martyrdom of his fellow Christians. A persecution broke out, and some of the leaders of the Lyons church were imprisoned; a few suffered martyrdom. This was in the reign of the philosophical pagan emperor, Marcus Aurelius. Since Lyons was a vital outpost of imperial power, adorned with temples and fine public buildings, the Roman officials perhaps thought it necessary to keep the new religion in check here. When Irenaeus returned from Rome it was to fill the now vacant bishopric. The brief period of persecution was over, and the twenty or more years of his episcopate were fairly peaceful. In addition to his pastoral duties at Lyons, Irenaeus is said to have extended the sphere of Christian influence by sending missionaries to other towns of Gaul-SS. Felix, Fortunatus, and Achilleus to Valence, and SS. Ferrutius and Ferreolus to Besancon. The bishop identified himself with his flock so completely as to speak habitually the native tongue instead of Latin or Greek, and to encourage all priests to do likewise.
The spread of Gnosticism in Gaul led Irenaeus to make a careful study of its tenets, not an easy matter since each Gnostic teacher was inclined to introduce subtleties of his own. He was, Tertullian tells us, "a curious explorer of all kinds of learning," and the task interested him. His treatise <Against the Heresies>, in five books, sets forth fully the doctrines of the main dissident sects of the day and then contrasts them with the words of Scripture and the teachings of the Apostles, as preserved not only in sacred writings but by oral tradition in the churches which the Apostles founded. Above all, he cites the authoritative tradition of the Church of Rome, handed down from Peter and Paul through an unbroken succession of bishops. In his theological works Irenaeus especially shows the influence of St. Paul and St. John. An humble, patient man, he writes of controversial matters with a moderation and courtesy unusual in this age of perfervid conviction.
An example of his method is his discussion of one type of Gnostic doctrine, that the visible world was created and is sustained and governed by angelic beings, but not by God, who remains unconnected with it, aloof and unmoved in his own inaccessible sphere. Irenaeus states the theory, develops it to a logical conclusion, and then by an effective <reductio ad absurdum> demonstrates its fallacy. The Christian doctrine of a close continuing relationship between the Triune God and the world He created Irenaeus describes thus: "The Father is above all, and He is the Head of Christ; the Word (Logos) is through all things and is Himself the Head of the Church, while the Spirit is in us all, and His is the living water which the Lord gave to those who believe in Him and love Him, and who know that there is one Father above all things and through all things." Irenaeus was convinced that the veil of mystery which enveloped Gnosticism was part of its attraction, and he was determined to "strip the fox," as he expressed it. His book, written in Greek and quickly translated into Latin, was widely circulated, and from this time on Gnosticism presented no serious threat.
Thirteen or fourteen years after his mission to Rome, Irenaeus attempted mediation between another Pope and a body of Christians in Asia Minor called the Quartodecimans, who refused to fix the day of Easter by the method commonly used by Christians. Pope Victor had excommunicated them, and Irenaeus pleaded with him in a beautiful letter to raise the ban, pointing out that these Asiatics were only following their Apostolic tradition, and that the difference of opinion on this minor point had not prevented St. Polycarp and many others from staying in communion. At the end of the fourth century Jerome wrote that many Eastern bishops still adhered to the ancient Jewish calendar.
The date of the death of Irenaeus is usually given as about the year 203. According to a late and dubious tradition he suffered martyrdom under Septimius Severus. His book <Against the Heresies> has come down to us entire in its Latin version; and an Armenian translation of his <Exposition of Apostolic Preaching> has lately been discovered. Though the rest of his writings have perished, in these two works may be found the elements of a complete system of Catholic theology.