St. Irenaeus, the bishop of Lugdunum (today Lyons, France) was born in Smyrna in Asia Minor, he was a pupil of Polycarp, who in turn, was a disciple of the apostle John. This connection to one of the original witnesses of Christ solidified the legitimacy of his teaching.
Two of Irenaeus’ many written works have come down to us in their entirety: “Against Heresies” and “Proof of Apostolic Preaching.” In the former work, Irenaeus attacks Gnosticism. Much of what he reported about Gnosticism could not be verified until the discovery of the Nag Hammadi (Egypt) library in 1945.
Gnostics asserted that matter is inherently evil and spirit is good. So, Gnostics believed anything done in the body, even the grossest sin, has no meaning because real life exists in the spirit realm only.
They did not accept the fusion of Christ’s divinity and humanity. Gnostics claimed to possess an elevated knowledge, a “higher truth” known only to a certain few. That knowledge did not come from the Bible, but was acquired on some mystical higher plane of existence. Gnostics saw themselves as a privileged class elevated above everybody else by their higher, deeper knowledge of God. Gnostics rejected the God of the Old Testament.
Irenaeus, on the contrary, used the Old Testament to substantiate his arguments. In “Proof of Apostolic Preaching,” he shows how Christ fulfilled prophecies of the Old Testament. He maintained that the bishops could trace their knowledge back to the apostles, who received their instruction from Jesus himself. He therefore established the bishops as the safeguards of the truth and, at his time, that was only a matter of 100 years of history. Thus, we have the foundation for the idea of apostolic succession.
Irenaeus work was widely known throughout Christendom even in his own time. There are some reports that he received a martyr’s death but there is no reliable proof of such.
Adapted by A.J Valentini from www.gotquestions.org/Irenaeus-of-Lyons.html