Romans 7:21

Paul says that the law of Moses agrees with his will against sin, which dwells in his flesh and forces him to do something other than what he and the law want to do.

For there is a certain weakness of this kind in those who receive the beginnings of conversion: that although one may wish to do at once everything that is good, the effect does not immediately follow the will. For a man, deliberating within himself, resolves (for example) not to grow angry, and he settles this in his will; but because by long use and lasting habit the vice of wrath has held dominion in him, it resists even his will and purpose, and, taking the path customary to it, the force of rage bursts out. The vice of lust acts in like manner. By an equal disease of habit a lie too creeps in, fear frightens him off; and in each of these that man who is now receiving the beginnings of conversion fittingly says that "to will is present with me, but to accomplish the good I find not. For the good that I will, this I do not; but the evil that I hate, that I do." But if the will holds the judgment of what is good, while the habit of carnal vices — which has been called the law of the flesh or the law of the members — resists and creeps in, then from the fact that I bear the will of the good, though I do evils, yet because I do them unwillingly, "I consent unto the law of God, that it is good." For I hate evils, and I desire to do the goods which the law commands. But if against my will I do evils, overcome by the habit of vices and by the practice of sinning, "it is no more I" — that is, the inner man — "who do the good I will; but sin that dwelleth in me." For he has named the habit of sinning "sin." "I find then a law, that, when I would do good, evil is present with me. For I delight in the law of God after the inward man." And in these words, as we have often noted above, we seem not to be fully unfolded in the expression, but to be hindered either by the abundance of the conjunctions or by their peculiarities. […]

Paul says that "evil lies close at hand" because our body is mortal and passible, and our soul is sluggish and weak.

What he says is not very clear. What then is it that is said? I praise the law, he says, in my conscience, and I find it pleads on my side so far as I am desirous of doing what is right, and that it invigorates this wish. For as I feel a pleasure in it, so does it yield praise to my decision. Do you see how he shows, that the knowledge of what is good and what is not such is an original and fundamental part of our nature, and that the Law of Moses praises it, and getteth praise from it? For above he did not say so much as I get taught by the Law, but "I consent to the Law;" nor further on that I get instructed by it, but "I delight in" it. Now what is "I delight?" It is, I agree with it as right, as it does with me when wishing to do what is good. And so the willing what is good and the not willing what is evil was made a fundamental part of us from the first. But the Law, when it came, was made at once a stronger accuser in what was bad, and a greater praiser in what was good. Do you observe that in every place he bears witness to its having a kind of intensitiveness and additional advantage, yet nothing further? For though it praises and I delight in it, and wish what is good the "evil is" still "present with me," and the agency of it has not been abolished. And thus the Law, with a man who determines upon doing anything good, only acts so far as auxiliary to him, as that it has the same wish as himself. Then since he had stated it indistinctly, as he goes on he gives a yet more distinct interpretation, by showing how the evil is present, how too the Law is a law to such a person only who has a mind to do what is good.

Even if the heretics who are opposed to the Creator suppose that in the next sentence Paul was speaking against him when he says, "I know that in me, that is in my flesh, there dwells no good thing," yet let them read what precedes and follows this. For before it he says, "But sin which dwells in me," which explains why it was appropriate for him to say, "in my flesh dwells no good thing." In what follows he continues, "But if I do that which I do not wish to do, it is no longer I that do it, but sin which dwells in me," which being at war with the law of God and "of my mind," he says, "makes me captive by the law of sin which is in my members. O wretched man that I am, who shall deliver me from this body of death." And again (for he does not become in the least weary of being helpful) he does not hesitate to add, "For the law of the Spirit has set me free from the law of sin and death," since by his Son "God condemned sin in the flesh that the righteousness of the law might be fulfilled in us who walk not after the flesh but after the Spirit." In addition to this he makes the point still clearer by saying emphatically, "The body is dead because of sin," indicating that if it is not the temple, it is still the tomb of the soul. For when it is dedicated to God, he adds, "the spirit of him who raised Jesus from the dead dwells in you, who shall also make alive your mortal bodies through his Spirit dwelling in you."