Irenaeus was an early Christian theologian and bishop, known for his role in shaping Christian doctrine in the second century. He is best known for his work, Against Heresies, which aimed to counter various heretical teachings and defend orthodox Christianity. Irenaeus emphasized the importance of the Apostolic Tradition and the authority of the Church Fathers. Born around 130 AD in Smyrna, Irenaeus later became the bishop of Lyons in modern-day France. His writings contributed significantly to the development of early Christian theology, particularly regarding the nature of God, Christ, and the Holy Spirit. Irenaeus is often regarded as a key figure in the establishment of the Christian canon.