18 All we therefore, who believe in the Living and True God, Whose Nature, being in the highest sense good and incapable of change, neither does any evil, nor suffers any evil, from Whom is every good, even that which admits of decrease, and Who admits not at all of decrease in His own Good, Which is Himself, when we hear the Apostle saying, “Walk in the Spirit, and perform ye not the lusts of the flesh. For the flesh lusts against the Spirit, and the Spirit against the flesh: For these are opposed one to another, that you do not what ye would.” Far be it from us to believe, what the madness of the Manichees believes, that there are here shown two natures or principles contrary one to another at strife, the one nature of good, the other of evil.
Altogether these two are both good; both the Spirit is a good, and the flesh a good: and man, who is composed of both, one ruling, the other obeying, is assuredly a good, but a good capable of change, which yet could not be made save by a Good incapable of change, by Whom was created every good, whether small or great; but how small soever, yet made by What is Great; and how great soever, yet no way to be compared with the greatness of the Maker. But in this nature of man, that is good, and well formed and ordered by One That is Good, there is now war, since there is not yet health.
Let the sickness be healed, there is peace. But that sickness fault has deserved, not nature has had. And this fault indeed through the laver of regeneration the grace of God has already remitted unto the faithful; but under the hands of the same Physician nature as yet strives with its sickness. But in such a conflict victory will be entire soundness; and that, soundness not for a time, but for ever: wherein not only this sickness is to come to an end, but also none to arise after it.
Wherefore the just man addresses his soul and says, “Bless the Lord, O my soul, and forget not all His returns: Who becomes propitious to all your iniquities, Who heals all your sicknesses.” He becomes propitious to our iniquities, when He pardons sins: He heals sicknesses when He restrains evil desires. He becomes propitious unto iniquities by the grant of forgiveness: He heals sicknesses, by the grant of continence. The one was done in Baptism to persons confessing; the other is done in the strife to persons contending; wherein through His help we are to overcome our disease.
Even now the one is done, when we are heard, saying, “Forgive us our debts;” but the other, when we are heard, saying, “Lead us not into temptation. For every one is tempted,” says the Apostle James, “being drawn away and enticed by his own lust.” And against this fault there is sought the help of medicine from Him, Who can heal all such sicknesses, not by the removal of a nature that is alien from us, but in the renewal of our own nature. Whence also the above-mentioned Apostle says not, “Every one is tempted” by lust, but added, “by his own:” that he who hears this may understand, how he ought to cry, “I said, Lord, have mercy upon me, heal my soul, for I have sinned against You.” For it would not have needed healing, had it not corrupted itself by sinning, so that its own flesh should lust against it, that is, itself should be opposed to itself, on that side, wherein in the flesh it was made sick.
Source: On Continence (New Advent)