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 (Is 7,14).
It was the Lord himself who saved them, for of themselves they had no power to be saved. (...)
Isaiah says the same: “Hands that are feeble grow strong! Knees that are week, take courage! Hearts that are faint grow strong! Fear not – see, our God is judgment and he will repay. He himself will come and save us (Is 35,3-4). He means that we could not be saved of ourselves but only with God's help.
Here is another text where Isaiah had predicted that he who saves us is not simply a man, nor an incorporeal being: “It was not a messenger or an angel, but he himself who saved them. Because of his love and pity he redeemed them himself” (Is 63,9). But this Savior is also really and truly a man, one our eyes will see: “Look on Zion, your eyes will see our Savior” (see Is 33,20). (...)
Another prophet said: “(He) will again have compassion on us...(he) will cast into the depths of the sea all our sins” (Mi 7,19). (...) It is from Bethlehem of Judea(Mi 5,1)that the Son of God, who is also God, was supposed to come to spread his praise all over the world (...) God really became man and the Lord himself saved us while giving us the sign of the Virgin.
- St. Irenaeus, Adversus Haereses 3.2.2