Furthermore, concerning these words of Job, “My heart shall not reproach me in all my life,” we remark, that it is in this present life of ours, in which we live by faith, that our heart does not reproach us, if the same faith whereby we believe unto righteousness does not neglect to rebuke our sin. On this principle the apostle says: “The good that I would I do not; but the evil which I would not, that I do.” Now it is a good thing to avoid concupiscence, and this good the just man would, who lives by faith; and still he does what he hates, because he has concupiscence, although “he goes not after his lusts;” if he has done this, he has himself at that time really done it, so as to yield to, and acquiesce in, and obey the desire of sin.
His heart then reproaches him, because it reproaches himself, and not his sin which dwells in him. But whenever he suffers not sin to reign in his mortal body to obey it in the lusts thereof, and yields not his members as instruments of unrighteousness unto sin, sin no doubt is present in his members, but it does not reign, because its desires are not obeyed. Therefore, while he does that which he would not—in other words, while he wishes not to lust, but still lusts—he consents to the law that it is good: for what the law would, that he also wishes; because it is his desire not to indulge concupiscence, and the law expressly says, “You shall not covet.” Now in that he wishes what the law also would have done, he no doubt consents to the law: but still he lusts, because he is not without sin; it is, however, no longer himself that does the thing, but the sin which dwells within him.
Hence it is that “his heart does not reproach him in all his life;” that is, in his faith, because the just man lives by faith, so that his faith is his very life. He knows, to be sure, that in himself dwells nothing good—even in his flesh, which is the dwelling-place of sin. By not consenting, however, to it, he lives by faith, wherewith he also calls upon God to help him in his contest against sin. Moreover, there is present to him to will that no sin at all should be in him, but then how to perfect this good is not present.
It is not the mere “doing” of a good thing that is not present to him, but the “perfecting” of it. For in this, that he yields no consent, he does good; he does good again, in this, that he hates his own lust; he does good also, in this, that he does not cease to give alms; and in this, that he forgives the man who sins against him, he does good; and in this, that he asks forgiveness for his own trespasses—sincerely avowing in his petition that he also forgives those who trespass against himself, and praying that he may not be led into temptation, but be delivered from evil—he does good.
But how to perfect the good is not present to him; it will be, however, in that final state, when the concupiscence which dwells in his members shall exist no more. His heart, therefore, does not reproach him, when it reproaches the sin which dwells in his members; nor can it reproach unbelief in him. Thus “in all his life,”— that is, in his faith—he is neither reproached by his own heart, nor convinced of not being without sin. And Job himself acknowledges this concerning himself, when he says, “Not one of my sins has escaped You; You have sealed up my transgressions in a bag, and marked if I have done iniquity unawares.” With regard, then, to the passages which he has adduced from the book of holy Job, we have shown to the best of our ability in what sense they ought to be taken.
He, however, has failed to explain the meaning of the words which he has himself quoted from the same Job: “Who then is pure from uncleanness? Not one; even if he be an infant of only one day upon the earth.”
Source: On Man's Perfection in Righteousness (New Advent)