3 For thus they will allege that this passage refers to the flesh strictly so called, and not to fleshly works, as I have pointed out, so representing the apostle as contradicting himself. For immediately following, in the same Epistle, he says conclusively, speaking thus in reference to the flesh: “For this corruptible must put on incorruption, and this mortal must put on immortality. So, when this mortal shall have put on immortality, then shall be brought to pass the saying which is written, Death is swallowed up in victory.
O death, where is your sting? O death, where is your victory?” Now these words shall be appropriately said at the time when this mortal and corruptible flesh, which is subject to death, which also is pressed down by a certain dominion of death, rising up into life, shall put on incorruption and immortality. For then, indeed, shall death be truly vanquished, when that flesh which is held down by it shall go forth from under its dominion. And again, to the Philippians he says: “But our conversation is in heaven, from whence also we look for the Saviour, the Lord Jesus, who shall transfigure the body of our humiliation conformable to the body of His glory, even as He is able (ita ut possit) according to the working of His own power.” What, then, is this “body of humiliation” which the Lord shall transfigure, [so as to be] conformed to “the body of His glory?”
Plainly it is this body composed of flesh, which is indeed humbled when it falls into the earth. Now its transformation [takes place thus], that while it is mortal and corruptible, it becomes immortal and incorruptible, not after its own proper substance, but after the mighty working of the Lord, who is able to invest the mortal with immortality, and the corruptible with incorruption. And therefore he says, “that mortality may be swallowed up of life. He who has perfected us for this very thing is God, who also has given unto us the earnest of the Spirit.” He uses these words most manifestly in reference to the flesh; for the soul is not mortal, neither is the spirit.
Now, what is mortal shall be swallowed up of life, when the flesh is dead no longer, but remains living and incorruptible, hymning the praises of God, who has perfected us for this very thing. In order, therefore, that we may be perfected for this, aptly does he say to the Corinthians, “Glorify God in your body.” Now God is He who gives rise to immortality.
Source: Adversus haereses (New Advent)