Irenaeus (c. n.D. 130-200) was born of Greek parents in Asia (modern-day Turkey). In his youth he learned under Polycarp, bishop of Smyrna, who had been a disciple of the Apostle John. As a teenager Irenaeus served as a missionary to Gaul (France), where he later filled the office of bishop. Some later Christian authorities indicate that he was martyred during the reign of Septimius Severus.
Irenaeus was the most important defender of essential Christian doctrine in the second century. He was especially effective in combating the heresy of Gnosticism. a cult that stressed secret knowledge (gnosis) as the way to salvation. Irenaeus contended with the gnostic leader Marcion, who taught that the OT and NT reveal two different gods. The god of the OT was the creator of matter, which, according to gnostic teaching, was evil. Thus, according to Marcion, the OT was useless. The NT God, as the God of law, unknown before the coming of Jesus Christ. Irenaeus powerfully refuted Marcion by demonstrating the interrelationship of the Testaments. He detailed how the OT accurately predicted the coming of Christ, Moreover, Irenaeus stressed the literal resurrection of Jesus. The Redeemer's physical body is not only not evil but in fact raised from the dead for our salvation – precisely as the OT prophets had predicted.
Extracted from the Apologetics Study Bible.