7:21 Paul confesses that he is carnal, sold unto sin (v. 14), that more specifically the problem is in his flesh in which nothing good dwells (v. 18) and now that evil (Gk. kakon) is present.
7:21 He finds a principle or law at work in his life causing all his good intentions to end in failure. When he wants to do what is right, he ends up by sinning.
21. So I discover this law: When I want to do good, evil lies close at hand.
The word “So” shows that the apostle here summarizes the contents of the preceding verses (14–20). It is immediately clear that when he here uses the word “law,” he is not thinking of the Ten Commandments. “Law,” as here used, must mean something like operating rule or governing principle. For more on this see verse 23.
The inflexible “law” to which reference is here made, and which the author of this epistle—as well as every believer—is constantly discovering, is this: “When I want to do good, evil lies close at hand.” In view of the fact that, according to verses 17, 20, sinful human nature has established its residence in Paul’s own house (his soul), and has done this with a wicked purpose, the statement “evil lies close at hand,” is indeed very logical. This “evil,” here personified, may be lying down, but is certainly not sleeping. It is pictured as if it were watching the apostle to see whether he is about to carry out a good intention. Whenever such a noble thought or suggestion enters Paul’s heart, evil immediately interrupts in order to turn the good deed into its opposite.
21 On the basis of the unsuccessful struggle to do the good demanded by the Mosaic law, Paul now draws a conclusion: “Therefore [Gk. ara] I find this law: when I will to do the good, evil is present there with me.” Consistency would suggest that the “law” (nomos) Paul refers to here is the Mosaic law, in accordance with his usual use of the term and its meaning throughout 7:4–20. We would then translate “I find, with respect to the law, that.…” But it makes better sense to give nomos here its well-established meaning “principle.”
21 The question on which interpreters are greatly divided is the denotation of “the law” in this verse, whether it refers to the law of God (vs. 22) or to the “other law”, “the law of sin” in our members (vs. 23). Either interpretation makes good sense and is both grammatically and syntactically acceptable. On the former view, namely, that “the law” refers to the law of God, the thought would be as follows: “For me willing conformity to the law in order to do the good I find that the evil is present with me”. Hence what he finds is that evil is present notwithstanding his determinate will to the good which the law of God requires. This fits in well with verse 22 in which he defines this determinate will to the good as delight in the law of God after the inward man. And it is also in accord with verse 23 where the opposing law of sin in his members is called “another law” in contrast with the law of God which, up to this point it is maintained, is the only law referred to in the passage. There is, however, no conclusive objection to the other interpretation, namely, that “the law of sin” (vss. 23, 25) is in view here and that it is defined in terms of the presence of evil in opposition to the determinate will to good. This is the view adopted in the version and, if followed, means that “law” in this instance is used in the sense of rule or principle of action. The usual signification of law, however, as that which propounds and demands action need not be suppressed. “The law of sin” may be conceived of as not only impelling to action that is antithetical to the law of God but also as dictating such action.
I find then the principle that evil is present in me, the one who wishes to do good. (7:21)
The continuing presence of evil in a believer’s life is so universal that Paul refers to it not as an uncommon thing but as such a common reality as to be called a continually operating spiritual principle. Lingering sin does battle with every good thing a believer desires to do, every good thought, every good intention, every good motive, every good word, every good deed.
The Lord warned Cain when he became angry that Abel’s sacrifice was accepted but his own was not: “Sin is crouching at the door; and its desire is for you, but you must master it” (Gen. 4:7). Sin continues to crouch at the door, even of believers, in order to lead people into disobedience.
 MacDonald, W. (1995). Believer’s Bible Commentary: Old and New Testaments. (A. Farstad, Ed.) (p. 1707). Nashville: Thomas Nelson.
 Moo, D. J. (2018). The Letter to the Romans. (N. B. Stonehouse, F. F. Bruce, G. D. Fee, & J. B. Green, Eds.) (Second Edition, pp. 483–484). Grand Rapids, MI: William B. Eerdmans Publishing Company.