Not Under Law, but Under Grace
Romans 6:14 is a great consolation for believers who feel like their salvation is based on their own righteousness, good deeds, or practical obedience.
"For sin will have no dominion over you, since you are not under law but under grace." (Romans 6:14)
"YOU ARE NOT UNDER LAW"
Now, if you were under law, then your salvation would be based on your own righteousness, good deeds, or practical obedience. If we were under law, then we would have to serve the sentence of our own crimes against God. If you were under law, and therefore still alienated from the grace of God in Christ, then sin still would have dominion over you.
Is that the kind of situation you want to be in? Why, then, are we so prone to feel, think, and act as if our salvation is based on us, i.e., based on how well we follow rules, how righteous we can be, how many good deeds we perform, how much practical obedience we can produce? Because of the pride that comes from not grasping or believing the nature of God's grace in Christ. You need to be humbled by the grace of God in the gospel of Jesus Christ to the point where you completely look away from yourself and turn to Christ in heaven for all things pertaining to Spiritual life.
It is good -- provided that you have the right motives and goals born of the Spirit -- that you long for more righteousness, good deeds, and practical obedience. It is right that you want the dominion of sin in your heart and mind to be further diminished that you might be further conformed to the likeness of Christ. But the answer is not law. The remedy is not rules. The power to kill sin and the power to cultivate holiness does not come to you by way of commandments and regulations.
This power from God comes to sinners only through the gospel of Christ crucified and risen, the only hope for weak and wounded sinners.
"And we all, with unveiled face, beholding the glory of the Lord, are being transformed into the same image from one degree of glory to another. For this comes from the Lord who is the Spirit." (2 Corinthians 3:18)
Jesus lives and reigns and is always accessible for grace and help in your time of need. He will help you by His Word; He will help you by His Spirit. Which brings us to those other phrases in Romans 6:14.
"FOR SIN WILL HAVE NO DOMINION OVER YOU, SINCE YOU ARE...UNDER GRACE"
The Righteousness of Christ
Beloved, since you are under grace, then your salvation is based on Christ and His righteousness, not your own. Which means, since you are one with Christ, and since Christ is perfectly righteous and an all-sufficient Savior, your salvation is settled. You are saved. Jesus completed the work to gain your salvation in His life, death, and resurrection. And Jesus applied the blessing of salvation to you personally when He changed your heart through the gospel by the power of the Holy Spirit. "It is finished." And it can never be diminished or robbed. It is Christ who saved you and it is Christ who guards you.
The Sacrificial Death of Christ
And since we are under grace through faith in Christ, then we do not have to serve the sentence of our crimes against God.
"He himself bore our sins in his body on the tree, that we might die to sin and live to righteousness. By his wounds you have been healed" (1 Peter 2:24).
"So Christ, having been offered once to bear the sins of many, will appear a second time, not to deal with sin but to save those who are eagerly waiting for him" (Hebrews 9:28).
"But he was pierced for our transgressions;he was crushed for our iniquities; upon him was the chastisement that brought us peace, and with his wounds we are healed" (Isaiah 53:5).
"I am the good shepherd. The good shepherd lays down his life for the sheep" (John 10:11), and,
"For this reason the Father loves me, because I lay down my life that I may take it up again.  No one takes it from me, but I lay it down of my own accord. I have authority to lay it down, and I have authority to take it up again. This charge I have received from my Father" (John 10:17–18).
Freedom in Christ
Part of our freedom in Christ is freedom from the threats and penalties of the law against our lawless desires and deeds. Child of God, if Jesus took all the punishment for all your sin that you will ever commit, how could there be any more punishment for you to face? How in the world could the condemnation and angry displeasure of God still hang over your head? There can be no more punishment reserved for you. There can be no condemnation hanging over you. Do not in pride resist this grace. Be free. Embrace, enjoy, and revel and rest in this glorious freedom of the grace in God through the crucified yet living Lord, Jesus Christ.
"There is therefore now no condemnation for those who are in Christ Jesus" (Romans 8:1).
"By this is love perfected with us, so that we may have confidence for the day of judgment, because as he is so also are we in this world.  There is no fear in love, but perfect love casts out fear. For fear has to do with punishment, and whoever fears has not been perfected in love" (1 John 4:17–18).
The Power of the Holy Spirit
Finally, since you are are under grace and reconciled to God by a living union with the exalted Lord, then sin is no longer your master, and sin will not have dominion -- i.e., ruling power and influence -- over you. This is the magnificent comfort of Romans 6:14:
"For sin will have no dominion over you, since you are not under law but under grace."
This is the exact situation we want to be in. Praise God for His glorious gift. Just as the gift of forgiveness is yours in Christ, the gift of being liberated from the ruling power of sin is also yours. The Holy Spirit has taken up residence in your heart, and by grace through faith, He is mightily killing sin and cultivating holiness within you for the glory of God. Look to Christ, trust in Christ, cry out to Christ. He is merciful. He is mighty. He is always there to help you.