Why does sin no longer have dominion over a Christian?
Sin no longer has dominion over a Christian because, as Romans 6:14 states, the believer is not under the Law but under Grace. The ritual and stipulations of Israel's Law and the demands of Judaism have been set aside. Grace, not license, now governs the believer's standing.
Les explains Romans 6:14, where Paul writes that sin, or the old Adam, shall not have dominion over you. The reason given is that you are not under the Law but under Grace. Les emphasizes the complete difference between Law and Grace: the believer is not under the ritual and stipulations of the Law of Israel, nor under the demands of Judaism, for all that has been set aside. He also cautions that Grace is not license.
For sin shall not have dominion over you: for ye are not under the law, but under grace.