From a treatise Against Heresies by Saint Irenaeus, bishop
The plan of redemption through the Incarnation
God is man’s glory. Man is the vessel which receives God’s action and all his wisdom and power.
Just as a doctor is judged in his care for the sick, so God is revealed in his conduct with men. That is Paul’s reason for saying: God has made the whole world prisoner of unbelief that he may have mercy on all. He was speaking of man, who was disobedient to God, and cast off from immortality, and then found mercy, receiving through the Son of God the adoption he brings.
If man, without being puffed up or boastful, has a right belief regarding created things and their divine Creator, who, having given them being, holds them all in his power, and if man perseveres in God’s love, and in obedience and gratitude to him, he will receive greater glory from him. It will be a glory which will grow ever brighter until he takes on the likeness of the one who died for him.
He it was who took on the likeness of sinful flesh, to condemn sin and rid the flesh of sin, as now condemned. He wanted to invite man to take on his likeness, appointing man an imitator of God, establishing man in a way of life in obedience to the Father that would lead to the vision of God, and endowing man with power to receive the Father. He is the Word of God who dwelt with man and became the Son of Man to open the way for man to receive God, for God to dwell with man, according to the will of the Father.
For this reason the Lord himself gave as the sign of our salvation, the one who was born of the Virgin, Emmanuel. It was the Lord himself who saved them, for of themselves they had no power to be saved. For this reason Paul speaks of the weakness of man, and says: I know that no good dwells in my flesh, meaning that the blessing of our salvation comes not from us but from God. Again, he says: I am a wretched man; who will free me from this body doomed to die? Then he speaks of a liberator, thanks to Jesus Christ our Lord.
Isaiah says the same: Hands that are feeble, grow strong! Knees that are weak, take courage! Hearts that are faint, grow strong! Fear not; see, our God is judgement and he will repay. He himself will come and save us. He means that we could not be saved of ourselves but only with God’s help.