Irenaeus of Lyons was one of the most important theologians of the second-century Church. Born in Asia Minor, he was shaped by the Christian communities of the apostolic age and later remembered hearing Polycarp of Smyrna, who had known the apostle John. Irenaeus eventually became bishop of Lyons in Gaul, where he served as a pastor, teacher, and defender of the faith.

He is best known for his work Against Heresies, written against the Gnostic movements that threatened the churches of his day. In it, Irenaeus defended the goodness of creation, the unity of the Old and New Testaments, the true incarnation of Christ, and the apostolic faith handed down in the churches. Standing close to the generation of the apostles, he remains a vital witness to early Christian doctrine and one of the great voices of the post-apostolic Church. 

“Moreover He fulfilled the promise made to Abraham, which God had promised him, to make his seed as the stars of heaven. For this Christ did, who was born of the Virgin who was of Abraham’s seed, and constituted those who have faith in Him lights in the world, and by the same faith with Abraham justified the Gentiles. For Abraham believed God, and it was counted unto him for righteousness. In like manner we also are justified by faith in God: for the just shall live by faith.

Now not by the law is the promise to Abraham, but by faith: for Abraham was justified by faith: and for a righteous man the law is not made. In like manner we also are justified not by the law, but by faith, which is witnessed to in the law and in the prophets, whom the Word of God presents to us.”

Irenaeus, Demonstration 35, trans. Robinson, 102–103. [bold added]