6 But it is one thing to fight well, which now is, when the strife of death is resisted; another thing not to have an adversary, which will then be, when death, “the last enemy,” shall be destroyed. For Continence also itself, when it curbs and restrains lusts, at once both seeks the good unto the immortality of which we aim, and rejects the evil with which in this mortality we contend. Of the one it is forsooth the lover and beholder, but of the other both the enemy and witness: both seeking what becomes, and fleeing what misbecomes.
Assuredly Continence would not labor in curbing lusts, if we had no wishes contrary to what is becoming, if there were no opposition on the part of evil lust unto our good will. The Apostle cries aloud, “I know,” says he, “that there dwells not in me, that is in my flesh, good. For to will lies near to me, but to accomplish good I find not.” For now good can be done, so far as that there be no assent given unto evil lust: but good will be accomplished, when the evil lust itself shall come to an end. And also the same teacher of the Gentiles cries aloud, “I take pleasure together with the law of God after the inner man: but I see another law in my members, warring against the law of my mind.”
Source: On Continence (New Advent)