St. Irenaeus lived in the Lyon area of what is now France in the second century. He survived the persecutions under the emperor Marcus Aurelius, but he is best known for writing his mammoth work Against Heresies refuting Gnosticism.
This work was the main source for everything we knew about Gnosticism until the Nag Hammadi scrolls were found late in the last century. Irenaeus asserted the importance of bishops, of the long stream of tradition dating back to the apostles, and the authority of Scripture. The New Testament was not finalized yet, and Irenaeus argued for the reliability of the four gospels as four different and authoritative pictures of the life of Jesus.
Irenaeus helped to shape the church, the New Testament, and the future of Christianity. He is a "Father of the Church" in the Roman Catholic tradition.