15 I do not understand what I do. For what I want to do I do not do, but what I hate I do. 16 And if I do what I do not want to do, I agree that the law is good. 17 As it is, it is no longer I myself who do it, but it is sin living in me. 18 For I know that good itself does not dwell in me, that is, in my sinful nature.[c] For I have the desire to do what is good, but I cannot carry it out. 19 For I do not do the good I want to do, but the evil I do not want to do—this I keep on doing. 20 Now if I do what I do not want to do, it is no longer I who do it, but it is sin living in me that does it. Romans 7:15-20
Paul rightly describes "sin" not just as an act, but rather, as also a nature within each of us.
"And when He (the Holy Spirit) has come, He will convict the world of sin, and of righteousness, and of judgment: of sin, because they do not believe in Me; . . . . " John 16: 8,9
The core of all sin is not believing in Jesus. Ever other sin is a manifestation of this.