14 For sin shall not have dominion over you, for you are not under law but under grace.
The first part of the verse is a promise.
law — a principle, not specifically the Jewish Law. 1 Corinthians 15:56 — The sting of death is sin; and the strength of sin is the law.
Because we are alive unto God (verse 13), sin shall not have dominion as it did under law.
The law says you are a sinner and must die. Grace says Christ died for your sins.
William R. Newell in Romans Verse by Verse wrote the following on the nature of grace. This may serve to set forth the glorious truth of our freedom from law and yieldedness to the Holy Spirit who produces holiness under grace.
1) Grace is God acting freely according to His own nature as Love; with no promises or obligations to fulfill; and acting of course, righteously — in view of the cross.
2) Grace, therefore, is uncaused in the recipient; its cause lies wholly in the Giver, in God.
3) Grace also is sovereign. Not having debts to pay, or fulfilled conditions on man’s part to wait for, it can act toward whom and how, it pleases. It can, and does, often, place the worst deserving in the highest favors.” — Barnhouse, pg. 143