21 That, therefore, the flesh lusts against the Spirit, that there dwells not in our flesh good, that the law in our members is opposed to the law of the mind, is not a mingling of two natures caused of contrary principles, but a division of one against itself caused through desert of sin. We were not so in Adam, before that nature, having listened to and followed its deceiver, had despised and offended its Creator: that is, not the former life of man created, but the latter punishment of man condemned.
From which condemnation when set free by Grace, through Jesus Christ, being free they contend with their punishment, having received not as yet full salvation, but already a pledge of salvation: but when not set free, they are both guilty by reason of sins, and involved in punishments. But after this life for the guilty there will remain for ever punishment for their crime: for the free there will no more remain for ever either crime or punishment: but the good substances, spirit and flesh, will continue for ever, which God, Who is good, and incapable of change, created good although capable of change.
But they will continue having been changed for the better, never from this time to be changed for the worse: all evil being utterly destroyed, both what man has unjustly done, and what he has justly suffered. And, these two kinds of evil perishing utterly, whereof the one is of iniquity going before, the other of unhappiness following after, the will of man will be upright without any depravity. There it will be clear and plain to all, what now many of the faithful believe, few understand, that evil is not a substance: but that, as a wound in a body, so in a substance, which has made itself faulty, it has begun to exist, when the disease has commenced, and ceases to exist in it, when the healing has been perfected.
Therefore, all evil having arisen from us, and having been destroyed in us, our good also having been increased and perfected unto the height of most happy incorruption and immortality, of what kind shall either of our substances be? Forasmuch as now, in this corruption and mortality, when as yet “the corruptible body weighs down the soul;” and, what the Apostle says, “the body is dead by reason of sin;” yet the same himself bears such witness unto our flesh, that is, to our lowest and earthly part, as to say, what I made mention of a little above, “No one ever hated his own flesh.” And to add straightway, “but nourishes and cherishes it, as also Christ the Church.”
Source: On Continence (New Advent)