Irenaeus, a bishop in Gaul sometime in the latter half of the second century is mainly known for his work Against Heresies circa 175-185. The title is actually Refutation and Overthrow of Knowledge, falsely so-called – thus the shorter title.
This work is a summary and brief history of all the heresies known by Irenaeus, focusing on Gnosticism. Indeed, Gnosticism was the dominant heresy of that time, even overshadowing the orthodox faith in the Egyptian region to some extent.
We learn from the author himself that he grew up in the faith and actually sat at the feet of Polycarp as a young boy(A.H. III.3,4). Eusebius gives us more from a letter of Irenaeus which no longer survives:
“For when I was a boy I saw you in lower Asia with Polycarp….I remember the events of that time more clearly than those of recent years. For what boys learn, growing with their mind, becomes joined with it; so that I am able to describe the very place in which the blessed Polycarp sat as he discoursed, and his goings out and his comings in, and the manner of his life, and his physical appearance, and his discourses to the people, and the accounts which he gave of his intercourse with John and with the others who had seen the Lord. And as he remembered their words, and what he heard from them concerning the Lord, and concerning his miracles and his teaching, having received them from eyewitnesses of the ‘Word of life,’ Polycarp related all things in harmony with the Scriptures…I listened to them attentively, noting them down, not on paper, but in my heart. And continually, through God’s grace, I recall them faithfully.” (E.H. V.20,5-7)
What begins as a refutation of Gnostic groups becomes something of a history of the Christian church up until his day. In fact, Eusebius leans upon Irenaeus to a great degree in his church history volume written 200 years later. Irenaeus gives us many details about Christianity during this period that might have otherwise been lost. For example, he recounts the succession of bishops in Rome from Peter and Paul to his day. This is done to combat a claim being made by several heretical leaders that they were in the rightful lineage of the apostles. He gives us the basis for a creed recited during his day (A.H. III.4,2). He cites passages from the four canonical gospels and from almost every other NT book.
Many scholars during the early years of the 20th century attacked Irenaeus and his description of these Gnostic groups, accusing him of exaggeration in order to make the Gnostics look far worse.
The discovery of Nag Hammadi in 1945 of several Gnostic writings dated from the second century (the Nag Hammadi Library) have proven that Irenaeus was, in fact, not making anything up, nor was he exaggerating.
Irenaeus served as the bishop of Lyons until 202 when it is thought he may have died during the persecution under Emperor Severus.