St Irenaeus came from Asia Minor, where as a young man he heard St Polycarp of Smyrna, who, Irenaeus tells us, had known the Apostle John. He became a priest in Lyons, in Gaul, and after the martyrdom of its bishop St Pothinus in 177 he was made bishop there himself.
In his day the Gnostic teachers, the followers of Valentinus above all, claimed a secret knowledge handed down to a few. Irenaeus called their teaching "knowledge falsely so called", the words of St Paul to Timothy, and that is the work's full title: The Detection and Overthrow of Knowledge Falsely So Called.
The first two books set out the Gnostic systems and show where they fail. The last three set out the faith from the Scriptures: the tradition the apostles handed to the bishops of the churches and that the churches still kept openly, the four Gospels and no more, and the teaching that Christ, the new Adam, sums up and renews in Himself the whole of human life.
Most of the Greek original is lost. The work survives almost entirely in an early Latin translation, with the last two books also in Armenian and parts of the Greek kept in later writers.