Paul was acutely aware of sin. "Those who are according to the flesh set their minds on the things of the flesh" (Ro 8:5). "Those who are in the flesh cannot please God" (:8). He made it clear that it wasn't just his opinion of what sin is. In fact, even though "sin was in the world . . . [it was] not imputed" (5:13) before the law. "Apart from the Law sin is dead" (7:8). "I would not have come to know sin except through the Law" (:7). It was a "commandment, which was to result in life" (:10) by way of a "ministry . . . in letters engraved on stones" (2Co 3:7). "The Law came in so that the transgression would increase" (Ro 5:20). Therefore "the Law is spiritual" (7:14).
This behavior is explained as man's "flesh with its passions and desires" (Gal 5:24). It could be said that man's human nature is physical, sensuous and earthy. "The spirit is willing but the flesh is weak" (Mt 26:41). Man's flesh is but "a wind that passes and does not return" (Ps 78:39). "All flesh is grass, and all its loveliness is like the flower of the field. The grass withers, the flower fades" (Isa 40:6-7). He continues with "the deeds of the flesh are evident, which are: immorality, impurity, sensuality, idolatry, sorcery, enmities, strife, jealousy, outbursts of anger, disputes, dissensions, factions, envying, drunkenness, carousing, and things like these" (Gal 5:19-21).
Paul concludes by saying "I know that nothing good dwells in me, that is, in my flesh" (Ro 7:18). "The heart is more deceitful than all else and is desperately sick; who can understand it?" (Jer 17:9). He states "no longer am I the one doing it, but sin which dwells in me" (Ro 7:17). Sin reigns "in your mortal body" (6:12) and is a "body of death" (7:24). "The Scripture has shut up everyone under sin" (Gal 3:22) "making me a prisoner of the law of sin which is in my members" (Ro 7:23). The "pleasures of sin" (Heb 11:25) "easily entangles" (12:1) and sin can become "master over you" (Ro 6:14). It has "lusts" (:12) and "coveting of every kind" (7:8). You can be "led into" (2Co 11:29) and "caught in any trespass" (Gal 6:1). Then you can "be hardened by the deceitfulness of sin" (Heb 3:13).