Irenaeus of Lyons, Against Heresies, 3.18

It is plainly demonstrated that the Word existed with God in the beginning and made all things, and that he was always present with the human race. In the last times, at the Father's appointed hour, this Word united himself with his own creature and became a passible man. That fact excludes every objection from those who ask: 'If he was born then, he did not exist before as Christ.' We prove that the Son of God did not begin to exist at that moment, since he existed always with the Father. When he became incarnate and was made man, he recapitulated the long history of humanity in himself, providing our salvation in a concise way, so that what we lost in Adam — namely, being according to the image and likeness of God — we might recover in Christ Jesus. (John 1:1–3; Romans 5:14–19)

Since it was impossible for man, once conquered and defeated through disobedience, to be remade and to obtain the victory of the Savior, and equally impossible for one who had fallen under sin to attain salvation, the Son — the Word of God — undertook both tasks. Existing eternally, he descended from the Father, became incarnate, descended even to death, and brought the dispensation of our salvation to completion. He exhorts us without doubt to believe in this, and says again: 'Do not say in your heart, "Who will ascend into heaven?" (that is, to bring Christ down) or "Who will descend into the abyss?" (that is, to bring Christ up from the dead).' He then adds: 'If you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved.' (Deuteronomy 30:12–14; Romans 10:6–9)

He also gives the reason why the Word of God did these things: 'For this purpose Christ both lived and died and rose again, so that he might be Lord of both the living and the dead.' Writing again to the Corinthians he says: 'We proclaim Christ crucified.' He adds: 'The cup of blessing that we bless — is it not a sharing in the blood of Christ?' Who shares with us in these things? Is it the one who is fabricated from above, a Christ stretched across the horizon — that is, the end — who formed their mother? Or is it the one from the virgin, Emmanuel, who ate butter and honey, of whom the prophet says: 'He is a man, and who can know him?' (Isaiah 7:14–15). This same one was announced by Paul: 'For I delivered to you as of first importance that Christ died for our sins according to the Scriptures, and that he was buried, and that he was raised on the third day according to the Scriptures.' (1 Corinthians 15:3–4)

It is therefore clear that Paul knows no other Christ except this one alone — the one who suffered, was buried, and rose again, who was also born and whom he calls a man. When he had said, 'If Christ is proclaimed as raised from the dead,' he supplied the reason for his incarnation: 'For since death came through a man, the resurrection of the dead also comes through a man.' Everywhere, in the passion of our Lord, in his humanity, and in his mortification, he uses the name Christ — as in: 'Do not destroy by your food the one for whom Christ died,' and again: 'Now in Christ Jesus you who once were far off have been brought near by the blood of Christ,' and again: 'Christ redeemed us from the curse of the law by becoming a curse for us, for it is written: "Cursed is everyone who hangs on a tree."' (Deuteronomy 21:23; Galatians 3:13). And again: 'The weak brother perishes through your knowledge, the one for whom Christ died' — signifying that it was not an impassible Christ who descended into Jesus, but Jesus Christ himself, who was passible, suffered for us, lay down and rose again, descended and ascended, the Son of God made the Son of Man, just as the name itself signifies. For in the name Christ is implied the one who anoints (the Father), the one anointed (the Son), and the anointing (the Spirit). The Father anointed, the Son was anointed, in the Spirit who is the anointing, just as the word says through Isaiah: 'The Spirit of the Lord is upon me, because he has anointed me' — signifying both the anointing Father, the anointed Son, and the anointing, which is the Spirit. (Isaiah 61:1; Luke 4:18)

The Lord himself makes manifest the one who suffered. When he asked his disciples, 'Who do people say that the Son of Man is?' and Peter answered, 'You are the Christ, the Son of the living God,' Jesus praised him: 'Flesh and blood did not reveal this to you, but my Father in heaven.' He thus made clear that the Son of Man is the Christ, the Son of the living God. From that point he began to show the disciples that he must go to Jerusalem, suffer many things from the priests, be rejected, be crucified, and rise on the third day. The very one whom Peter recognized as Christ — whom he called blessed because the Father had revealed the Son of the living God to him — declared that he himself must suffer and be crucified. Then he rebuked Peter, who was thinking by human opinion that Jesus was the Christ while denying his passion, and said to the disciples: 'If anyone wants to follow me, let him deny himself, take up his cross, and follow me. For whoever wants to save his life will lose it, but whoever loses his life for my sake will save it.' Christ said these things plainly, himself being the Savior of those who would be handed over to death for his confession and would lose their lives. (Matthew 16:13–25)