20 There are therefore in us evil desires, by consenting not unto which we live not ill: there are in us lusts of sins, by obeying not which we perfect not evil, but by having them do not as yet perfect good. The Apostle shows both, that neither is good here perfected, where evil is so lusted after, nor evil here perfected, whereas such lust is not obeyed. The one forsooth he shows, where he says, “To will is present with me, but to perfect good is not;” the other, where he says, “Walk in the Spirit, and perfect not the lusts of the flesh.” For neither in the former place does he say that to do good is not with him, but “to perfect,” nor here does he say, Have not “lusts of the flesh,” but “perfect not.”
Therefore there take place in us evil lusts, when that pleases which is not lawful; but they are not perfected, when evil lusts are restrained by the mind serving the Law of God. And good takes place, when that, which wrongly pleases, takes not place through the good delight prevailing. But the perfection of good is not fulfilled, so long as by the flesh serving the law of sin, evil lust entices, and, although it be restrained, is yet moved. For there would be no need for it to be restrained, were it not moved.
There will be at some time also the perfection of good, when the destruction of evil: the one will be highest, the other will be no more. And if we think that this is to be hoped for in this mortal state, we are deceived. For it shall be then, when death shall not be; and it shall be there, where shall be life eternal. For in that world, and in that kingdom, there shall be highest good, no evil: when there shall be, and where there shall be, highest love of wisdom, no labor of continence.
Therefore the flesh is not evil, if it be void of evil, that is, of fault, whereby man was rendered faulty, not made ill, but himself making. For on either part, that is, both soul and body, being made good by the good God, himself made the evil, whereby he was made evil. From the guilt of which evil being already also set free through forgiveness, that he may not think what he has done to be light, he yet wars with his own fault through continence. But far be it that there be any faults in such as reign in that peace which shall be hereafter; since in this state of war there are lessened daily in such as make progress, not sins only, but the very lusts also, with which, by not consenting, we strive, and by consenting unto which we sin.
Source: On Continence (New Advent)