Romans 7:18

18.For I know, etc. He says that no good by nature dwelt in him. Then in me, means the same as though he had said, “So far as it regards myself.” In the first part he indeed arraigns himself as being wholly depraved, for he confesses that no good dwelt in him; and then he subjoins a modification, lest he should slight the grace of God which also dwelt in him, but was no part of his flesh. And here again he confirms the fact, that he did not speak of men in general, but of the faithful, who are divided into two parts — the relics of the flesh, and grace. For why was the modification made, except some part was exempt from depravity, and therefore not flesh? Under the term flesh, he ever includes all that human nature is, everything in man, except the sanctification of the Spirit. In the same manner, by the term spirit, which is commonly opposed to the flesh, he means that part of the soul which the Spirit of God has so re-formed, and purified from corruption, that God’s image shines forth in it. Then both terms, flesh as well as spirit, belong to the soul; but the latter to that part which is renewed, and the former to that which still retains its natural character. (227)

To will is present, etc. He does not mean that he had nothing but an ineffectual desire, but his meaning is, that the work really done did not correspond to his will; for the flesh hindered him from doing perfectly what he did. So also understand what follows, The evil I desire not, that I do: for the flesh not only impedes the faithful, so that they can not run swiftly, but it sets also before them many obstacles at which they stumble. Hence they do not, because they accomplish not, what they would, with the alacrity that is meet. This, to will, then, which he mentions, is the readiness of faith, when the Holy Spirit so prepares the godly that they are ready and strive to render obedience to God; but as their ability is not equal to what they wish, Paul says, that he found not what he desired, even the accomplishment of the good he aimed at.