Born around 125, Irenaeus was sent to Lyon in the south-central part of the province of Gallia, modern day France. There, though late in his career at the age of 55, he helped establish a beachhead for the growing Christian presence in Gaul, furthering the work of Pothinus and others who had gone before.

Irenaeus spoke Greek as his native tongue, and confessed that he learned the Celtic dialect of Gaul with difficulty. Though of little eloquence, this did not prevent him from preparing the most complete catholic account of the faith written to his time, providing not the vain and heady speculations of his Gnostic opponents Valentinus or Marcion, but a solid and nuanced approach to the doctrines of the faith.

His works remain only in tattered pieces. Most letters are lost, as well as his shorter treatises on Greek philosophy and narrative accounts of the persecutions afflicting the Gallic churches in his day. What has remained are his five books refuting Gnosticism, written between 177 and 190. Although a Greek copy, the language in which it was written, it attested as late as the sixteenth century, all we have now is a clumsy and literal Latin translation. Thankfully this is sufficient to give us a clear idea of his teachings and doctrines.

In 140, two persuasive teachers were gaining in popularity at Rome, Valentinus and Marcion whom I have mentioned. Though the two differed in some respects, they shared an emphasis on the importance of developing speculative knowledge and the denigration of the Scriptures as mere folklore and fable. These concerns eventuated in a complex series of doctines which taught that the material world was made not by God the Creator, but by a Demiurge (a notion misappropriated from Platonic philosophy) who could freely mingle with matter and not be contaminated. God himself, they said, must remain aloof from the crassness of all physical interaction. Gnostic doctrines breathed an air of scientific sophistication, and provided for a three-tiered system of human society through which one could move by acquiring special knowledge. The first level was that of the somatic, the state characterized only by a stupid and drone-like focus on physical needs. Those who had been enlightened by philosophy were the psychical men, having their reason refined and perfected. By employing the tenets of Gnostic learning, Valentinus taught, one may advance to the pneumatic man, the life of the spirit in which all bodily interactions are despised and almost unnoticed.

When a friend asked Irenaeus to write against these teachings, which were quickly infiltrating the churches of southern Gaul, the bishop realized both that these heady speculations served to flatter human vanity, and that they were opposed to the plain teachings of the Scriptures. The occasion of this heresy, in God's singular providence, called forth from Irenaeus a dazzling defense of orthodoxy.

Irenaeus recognized that the core of Gnosticism was not the work of God in Jesus Christ, but the acquisition of secret knowledge. Confession of sin, the inward work of regeneration, repentance and self-denial played little part in the Gnostic teaching. Instead Valentinus and his associates taught that one must learn the order of the heavens, the grossness of physical creation, and a quasi-magical set of formulas. In sharp contrast, Irenaeus wrote: "The true way to God is love. It is better to be willing to know nothing but Jesus Christ the crucified, than to fall into ungodliness through over-curious questions and paltry subtleties" (Schaff 2.13).

Though not averse to Greek learning (Irenaeus could quote Homer and Plato with ease), the father grounded his doctrines on Scripture, and a mature reading of them. As Schaff says, Irenaeus was the first writer we have who utilizes the New Testament canon in its fullness. Though the Apostles' immediate successors report oral traditions, and individuals like Justin Martyr quote the Old Testament, Irenaeus alone showed the close inter-reliance of the two testimonies. In his reading of the New Testament, we find not the pick-and-choose method of Marcion heresy, but a full selection from all four Gospels and nearly every Epistle.

Irenaeus also anticipates the important doctrine of organic inspiration. Writing in Book III of Refutation of Gnosticism, he shows that the Gnostics do not know how to read Paul in II Corinthians because they are ignorant of Paul's style and customary manner of speaking:

Paul does not say, "the God of this world," as if recognizing any other beyond Him; but he confessed God as indeed God. And he says, "the unbelievers of this world," because they shall not inherit the future age of incorruption. I shall show from Paul himself, how it is that God has blinded the minds of them that believe not, in the course of this work, that we may not just at present distract our mind from the matter in hand, [by wandering] at large. From many other instances also, we may discover that the apostle frequently uses a transposed order in his sentences, due to the rapidity of his discourses, and the impetus of the Spirit which is in him." (III.7.2)

The contributions of Irenaeus to catholic doctrine are too numerous to list, but we may also mention his focus on inner spirituality over the asceticism and legalism that was rising at the time in the Montanistic controversy, as well as his teaching on proper church structure and government. When heretics would dodge his attacks and change their opinions readily, Irenaeus would calmly return to the biblical teachings and show that indeed all true believers had always grounded their faith upon them.

If you wish to explore the writings of Irenaeus further, may I encourage you to consult the following resources online:

http://www.earlychristianwritings.com/irenaeus.html

http://www.ccel.org/fathers2/ANF-01