Now if I do that I would not, it is no more I that do it, but sin that dwelleth in me.

People's New Testament

7:20 Now if I do that which I would not, etc. This experience sustains Ro 7:17 and shows that sin had predominated over human nature and rules it. Sin controls, rather than good intentions. A man wills one thing and does another.

Scofield Reference Notes

Margin sin

Sin. See Scofield Note: "Rom 5:21".

Matthew Henry's Concise Commentary

7:18-22 The more pure and holy the heart is, it will have the more quick feeling as to the sin that remains in it. The believer sees more of the beauty of holiness and the excellence of the law. His earnest desires to obey, increase as he grows in grace. But the whole good on which his will is fully bent, he does not do; sin ever springing up in him, through remaining corruption, he often does evil, though against the fixed determination of his will. The motions of sin within grieved the apostle. If by the striving of the flesh against the Spirit, was meant that he could not do or perform as the Spirit suggested, so also, by the effectual opposition of the Spirit, he could not do what the flesh prompted him to do. How different this case from that of those who make themselves easy with regard to the inward motions of the flesh prompting them to evil; who, against the light and warning of conscience, go on, even in outward practice, to do evil, and thus, with forethought, go on in the road to perdition! For as the believer is under grace, and his will is for the way of holiness, he sincerely delights in the law of God, and in the holiness which it demands, according to his inward man; that new man in him, which after God is created in true holiness.