Why will sin (old Adam) not have dominion over believers?
Sin, or old Adam, will not have dominion over believers because they are not under law but under grace. God introduced the gospel of grace, setting Gentiles free from legalism, and believers are saved by faith in Christ's death and resurrection plus nothing.
Les explains that when God introduced the gospel of grace, Gentiles were no longer subject to the Judaic system or its legalism. Believers are set free from that. Salvation is simply believing that Christ died for them and rose from the dead, plus nothing. However, this does not mean stopping there; believers are created unto good works. Paul teaches that we should not give in to old Adam but live above his desires. We have the indwelling Holy Spirit, and Christ Himself becomes the head of our daily living. Therefore, old Adam will not have dominion over you because you are not under law but under grace. For over 1900 years, God has dealt with the whole human race on the basis of His grace, though few understand it.
For sin shall not have dominion over you: for ye are not under the law, but under grace.