V. Christ's flesh is proved real from Scripture.
Therefore in consequence of this unity of person which is to be understood in both natures, we read of the Son of Man also descending from heaven, when the Son of God took flesh from the Virgin who bore Him. And again the Son of God is said to have been crucified and buried, although it was not actually in His Divinity whereby the Only-begotten is co-eternal and con-substantial with the Father, but in His weak human nature that He suffered these things. And so it is that in the Creed also we all confess that the Only-begotten Son of God was crucified and buried, according to that saying of the Apostle: “for if they had known, they would never have crucified the Lord of glory.”
But when our Lord and Saviour Himself would instruct His disciples' faith by His questionings, He said, “Whom do men say that I, the Son of Man, am?” And when they had put on record the various opinions of other people, He said, “But you, whom do you say that I am?” Me, that is, who am the Son of Man, and whom you see in the form of a slave, and in true flesh, whom do you say that I am? Whereupon blessed Peter, whose divinely inspired confession was destined to profit all nations, said, “You are Christ, the Son of the living God.”
And not undeservedly was he pronounced blessed by the Lord, drawing from the chief corner-stone the solidity of power which his name also expresses, he, who, through the revelation of the Father, confessed Him to be at once Christ and Son of God: because the receiving of the one of these without the other was of no avail to salvation, and it was equally perilous to have believed the Lord Jesus Christ to be either only God without man, or only man without God. But after the Lord's resurrection (which, of course, was of His true body, because He was raised the same as He had died and been buried), what else was effected by the forty days' delay than the cleansing of our faith's purity from all darkness?
For to that end He talked with His disciples, and dwelt and ate with them, He allowed Himself to be handled with diligent and curious touch by those who were affected by doubt, He entered when the doors were shut upon the Apostles, and by His breathing upon them gave them the Holy Spirit, and bestowing on them the light of understanding, opened the secrets of the Holy Scriptures. So again He showed the wound in His side, the marks of the nails, and all the signs of His quite recent suffering, saying, “See My hands and feet, that it is I.
Handle Me and see that a spirit has not flesh and bones, as you see Me have;” in order that the properties of His Divine and human nature might be acknowledged to remain still inseparable: and that we might know the Word not to be different from the flesh, in such a sense as also to confess that the one Son of God is both the Word and flesh. Of this mystery of the faith your opponent Eutyches must be reckoned to have but little sense if he has recognized our nature in the Only-begotten of God neither through the humiliation of His having to die, nor through the glory of His rising again.
Nor has he any fear of the blessed apostle and evangelist John's declaration when he says, “every spirit which confesses Jesus Christ to have come in the flesh, is of God: and every spirit which destroys Jesus is not of God, and this is Antichrist.” But what is “to destroy Jesus,” except to take away the human nature from Him, and to render void the mystery, by which alone we were saved, by the most barefaced fictions. The truth is that being in darkness about the nature of Christ's body, he must also be befooled by the same blindness in the matter of His sufferings.
For if he does not think the cross of the Lord fictitious, and does not doubt that the punishment He underwent to save the world is likewise true, let him acknowledge the flesh of Him whose death he already believes: and let him not disbelieve Him man with a body like ours, since he acknowledges Him to have been able to suffer: seeing that the denial of His true flesh is also the denial of His bodily suffering. If therefore he receives the Christian faith, and does not turn away his ears from the preaching of the Gospel: let him see what was the nature that hung pierced with nails on the wooden cross, and, when the side of the Crucified was opened by the soldier's spear, let him understand whence it was that blood and water flowed, that the Church of God might be watered from the font and from the cup. Let him hear also the blessed Apostle Peter, proclaiming that the sanctification of the Spirit takes place through the sprinkling of Christ's blood. And let him not read cursorily the same Apostle's words when he says, “Knowing that not with corruptible things, such as silver and gold, have you been redeemed from your vain manner of life which is part of your fathers' tradition, but with the precious blood of Jesus Christ as of a lamb without spot and blemish.” Let him not resist too the witness of the blessed Apostle John, who says: “and the blood of Jesus the Son of God cleanses us from all sin.” And again: “this is the victory which overcomes the world, our faith.” And “who is He that overcomes the world save He that believes that Jesus is the Son of God. This is He that came by water and blood, Jesus Christ: not by water only, but by water and blood. And it is the Spirit that testifies, because the Spirit is the truth, because there are three that bear witness, the Spirit, the water and the blood, and the three are one.” The Spirit, that is, of sanctification, and the blood of redemption, and the water of baptism: because the three are one, and remain undivided, and none of them is separated from this connection; because the Catholic Church lives and progresses by this faith, so that in Christ Jesus neither the manhood without the true Godhead nor the Godhead without the true manhood is believed in.
Source: Letters (New Advent)