Irenaeus devotes Book IV of Against Heresies to proving from the Lord's own words that the God of the Law and the prophets is the same Father whom Jesus Christ proclaimed, refuting Marcion's split between the Creator and the Savior.
Book IV is the third installment of Irenaeus's five-book refutation of Gnostic and Marcionite teaching, written around 180 CE. Where Books I and II exposed and mirrored the Valentinian system and Book III argued the point from the testimony of the apostles, Book IV turns to the sayings of the Lord himself, marshaling parables, prophetic citations, and Gospel sayings to show that Jesus consistently acknowledged one God and Father — the same Maker who called Abraham, gave the Law to Moses, and spoke through the prophets. Irenaeus is arguing chiefly against Marcion, who had cut the New Testament God of grace away from the Old Testament God of the Law, and against Valentinian teachers who located the Demiurge below the true, unknowable Father.
The book unfolds as a long, patient chain of demonstrations rather than a single sustained argument. Irenaeus works through the parable of the vineyard and its wicked farmers, the calling of Abraham and his inheritance of the kingdom, the reason Israel took vessels from the Egyptians at the Exodus, the meaning of the tabernacle and its furnishings, and the place of the Law and the prophets as preparatory instruction for a still-immature humanity. Running through all of it is his signature idea that God's dealings with Israel were a graduated pedagogy: Adam was formed not because God needed anything but so that God would have someone to bless, and the covenants that follow — with Noah, Abraham, Moses, and finally in the Gospel — are stages by which humanity is trained and matured toward fellowship with God, culminating in the Incarnation and the gift of the Spirit.
Historically, Book IV is one of the fullest surviving second-century arguments for the unity of the two Testaments and against the docetic splitting of Christ from the Creator, making it a primary witness to how the emerging church defended a unified biblical canon and a coherent doctrine of God against rival Christian movements before either was formally settled. It also preserves, in the sayings attributed to 'the presbyter,' fragments of an earlier generation of Christian teaching that would otherwise be lost, and it develops Irenaeus's influential notion of progressive revelation — that Scripture's harshest or strangest commands must be read as accommodations to the moral capacity of the people receiving them at the time.
“In sending you, my dearest friend, this fourth book of the work concerning the detection and overthrow of the falsely called knowledge, we shall, as we promised, confirm through the sayings of the Lord the things we have already stated”
— Book IV — Preface
“In the beginning, God formed Adam not because he needed man, but so that he might have someone on whom to bestow his benefits.”
— Book IV — Chapter 25
“A certain man, master of an estate, laid out a vineyard, put a fence around it, dug within it a wine-vat, raised a watchtower over it, leased it to farmers, and journeyed abroad.”
— Book IV — Chapter 58
“Those who reproach and charge that the people, about to depart at God's command, received vessels of every kind and garments from the Egyptians and so departed, and that the tabernacle in the wilderness was made out of these — such people, ignorant of the just dealings of God and his dispensations, condemn themselves, as the presbyter used to say.”
— Book IV — Chapter 46
“'I have given you milk, not solid food, for you were not yet able to receive it' — that is, you had learned the coming of the Lord according to the flesh, but the Spirit of the Father did not yet rest upon you, because of your weakness.”
— Book IV — Chapter 63
Irenaeus argues that Jesus, in his own recorded sayings and parables, consistently identified his Father as the very God who created the world, gave the Law to Moses, and spoke through the prophets. This directly refutes Marcion and the Valentinians, who separated the Old Testament Creator from the New Testament Father of Jesus.
His chief targets are Marcion, who rejected the Old Testament God as inferior to the God revealed by Christ, and the Valentinian Gnostics, who placed the Demiurge (the world's maker) below a higher, unknowable Father. Irenaeus treats both as variants of the same basic error of splitting the Godhead.
He presents these as part of a graduated pedagogy: God trains humanity in stages appropriate to its capacity at each point in history, from Adam through Noah and Abraham to Moses, before humanity was ready for the fuller revelation in Christ. The Law's provisions, including the plundering of Egyptian goods and the tabernacle's construction, are read as fitting and just for their time, not as embarrassing anomalies.
Irenaeus cites an unnamed elder several times as an authority, most likely preserving teaching handed down from a disciple of the apostles such as Polycarp. These citations are valued by historians as fragments of otherwise lost early Christian tradition embedded within Irenaeus's own text.
He reads the parable's vineyard owner, who plants, fences, and eventually sends his own son to unfaithful farmers, as Christ's own testimony that the one who owns the vineyard and the one who sends the Son are the same Father — directly countering the claim that the Law-giving God and the Father of Jesus are different beings.
Irenaeus points to Abraham's justification by faith, prior to the Law and to circumcision, as proof that Gentile believers who have faith in Christ are Abraham's true heirs alongside the patriarchs, unifying old and new covenant peoples under a single promise rather than two separate economies.
It is one of the fullest surviving second-century defenses of the unity of the two Testaments and a unified doctrine of God, offering historians a detailed picture of how mainstream Christianity argued against rival Christian movements before the canon or later creeds were fixed.
Irenaeus wrote Against Heresies (Adversus Haereses) in Greek around 180 CE while serving as bishop of Lyon; the complete text survives mainly in an early Latin translation, with substantial Greek fragments preserved by later writers.
Against Heresies — Book I · Against Heresies — Book II · Against Heresies — Book III · Against Heresies — Book V