Saint Irenaeus was a Christian bishop and theologian who lived in the second century AD. He is best known for his work against heresies, particularly in his book, Against Heresies, where he defended orthodox Christian beliefs against Gnostic teachings. Irenaeus emphasized the importance of the Apostolic Tradition and the role of the Church in preserving the true faith. Irenaeus was born in Smyrna (modern-day Izmir, Turkey) and later became the bishop of Lyons in France. His writings contributed significantly to early Christian theology and helped shape the understanding of the nature of God and the role of Jesus Christ. He is recognized as a saint in both the Roman Catholic Church and the Eastern Orthodox Church.