If the old man/sin nature remains then Christ died in vain and there remains no more of getting rid of the old man/sin nature. "For if we send willfully after that we have received the knowledge of the truth, the remaining no more sacrifice for sins," Hebrews 10:26. The word willfully means that it pertains to the mind because the will is residence within the soul/mind area. This means that it is a sin originating they committed through the use of the soul/mind and body along well totally ignoring the "Christ in you" nature in the believer. Those under the law knew that man sinned and could never reach the place where God was totally pleased with them. No one prior to Paul except Jesus knew that the problem with mankind was that mankind had a sin-nature\spirit. They never knew that the problem was a sin nature and that this is why they committed sins. After the salutation Paul begins the Romans epistle by writing about the greatest problem humanity has as result of the old sin nature residing within them. In the first three chapters Paul writes about the greatest manifestation of the old sin nature which causes corruption of the mind that is in union/marriage with the sin nature. Immediately after the salutation Paul delves into the fact that humanity has a "mind" problem that will continuously get worse if unimpeded. In the first section of Romans Paul is not dealing with the source of mankind's problems but is dealing with how the sin nature manifest itself through compatibility with the soul/mind of human, Romans chapters 1–3. What this means is Paul begins by letting the readers know about the corrupt mind that all mankind has.