Let not sin therefore reign in your mortal body, to make you obey its passions. Do not present your members to sin as instruments for unrighteousness, but present yourselves to God as those who have been brought from death to life, and your members to God as instruments for righteousness. For sin will have no dominion over you, since you are not under law but under grace.
Paul's basis for giving this command seen in Romans 6:12, "Let not sin reign..." is founded upon verses 6-11 which tell of how Jesus defeated sin in His death and therefore since we were "baptized" with Him in His death, we are dead to sin, and we ought to "reckon" ourselves dead to sin. When Jesus was resurrected, He showed that He had defeated the power of sin and death and the one who believes in Jesus is also free from this enslavement to sin and death. Through Christ's death and resurrection He has absorbed the wrath of God and the borne the punishment for sinners, and He has broken the fetters of sin and death for those who believe. Therefore sin has no power over the one who trusts in Jesus's work.
So Paul's command in Romans 6:12 is not a command to dethrone sin in our own power. Rather, Paul tells his readers to keep dethroning sin because it has been dethroned. Jesus has defeated sin once and for all and so sin has absolutely no power over the one who trusts in Jesus. The command "let not sin reign.." is based upon Jesus already having defeated sin. This gives us rest.
Romans 6:12-13 presents a real command and responsibility on our part as patients resurrected by the Doctor, to not return to the sin that enslaved us and trapped us in spiritual death. Everyday we must fight sin passionately, resisting temptation, wrestling with the flesh, and violently cutting off ourselves from sinning. But this war on our sin is not the grounds for our salvation. Our work in fighting sin is not the merit that earns us righteousness. Jesus's once and for all cancellation of sin and death is the rock upon which our righteousness and salvation rests. So the only way that we are able to fight is because He has fought.
When I had previously read Romans 6:14, I had always believed that this verse was Paul simply stating a fact, or giving encouragement to the believer to execute this command he just gave, but recently I have found that this statement "For sin will have no dominion over you, since you are not under law but under grace.", is a promise, rather than just a "good luck, homie...Im praying for you" encouraging sentence. I guess what I believed previously was that this was Paul's encouraging statement, but recently I have seen that this is the Almighty God's promise. Sin WILL NOT have dominion over you! It came off the pages as a protective, Fatherly sounding promise that says "My sheep hear my voice, and I know them, and they follow me. I give them eternal life, and they will never perish, and no one will snatch them out of my hand.My Father, who has given them to me, is greater than all, and no one is able to snatch them out of the Father's hand. (John 10:27-29)
Furthermore, after reading this promise, I began reading these three verses backwards in my head, saying "Sin will not have dominion over you, because you are not under the law but under grace, therefore let not sin reign in your mortal body..." Sin will not win the war because Christ has already won the war, so go fight! I am an avid basketball fan, and I enjoy playing far more than watching, so when I read this, an image came to mind. The parallels will not match everywhere, but you'll get the point. Imagine that a team is in its locker room before the final game of its season. They are nervous in facing an opponent that is highly ranked and greatly feared. Just before the game, God Himself walks into the locker room and informs the coach and the players that they will win by 15 points, says "This team will not beat you, now go play", and then walks out. This would be way more than encouraging to the players in the locker room and it would completely remove any stress, tension or worry about what the outcome of the game would be. I think God does way more for us when it comes to us fighting sin, and this was a very trite illustration, but I think this is generally what is happening in Romans 6:12-14. The promise takes the burden off of the command.
Thinking more about these verses in Romans 6, I was looking for confirmation of this promise/command thing in other places in Scripture. I came across several Old Testament passages where this pattern of promise and command happened for God's people in many instances of war and battle.
Deuteronomy 2:30-33 states "But Sihon the king of Heshbon would not let us pass by him, for the LORD your God hardened his spirit and made his heart obstinate, that he might give him into your hand, as he is this day. And the LORD said to me, ‘Behold, I have begun to give Sihon and his land over to you. Begin to take possession, that you may occupy his land.’ Then Sihon came out against us, he and all his people, to battle at Jahaz. And the LORD our God gave him over to us, and we defeated him and his sons and all his people."
In these verses, the promise from God is given to Israel before the battle takes place. "I have begun to give Sihon and his land over to you." is the promise from God to His people that Sihon didn't stand a chance against them. Next is the Lord's command to them to "begin to take possession, that you may occupy his land."
Deuteronomy 3:1-3 was another passage that confirmed the same thing.
"Then we turned and went up the way to Bashan. And Og the king of Bashan came out against us, he and all his people, to battle at Edrei. But the LORD said to me, ‘Do not fear him, for I have given him and all his people and his land into your hand. And you shall do to him as you did to Sihon the king of the Amorites, who lived at Heshbon.’ So the LORD our God gave into our hand Og also, the king of Bashan, and all his people, and we struck him down until he had no survivor left. "
Here, another king, Bashan, comes out against Israel and the Lord promises His people that "I have given him and all his people and his land into your hand." The command to "do to him as you did to Sihon" follows and once again Israel has success. When God makes these promises before the battle occurs in these verses, from His reality and perspective these enemies of Israel have already been defeated. This is the same reality that is true with the Christians war on sin. "Sin shall have no dominion over you, it has been defeated in Jesus' death and resurrection, so kill it."
One last passage in the Old Testament that displays the same pattern is Joshua 10:8-11 which states
And the LORD said to Joshua, “Do not fear them, for I have given them into your hands. Not a man of them shall stand before you.” So Joshua came upon them suddenly, having marched up all night from Gilgal. And the LORD threw them into a panic before Israel, who struck them with a great blow at Gibeon and chased them by the way of the ascent of Beth-horon and struck them as far as Azekah and Makkedah. And as they fled before Israel, while they were going down the ascent of Beth-horon, the LORD threw down large stones from heaven on them as far as Azekah, and they died. There were more who died because of the hailstones than the sons of Israel killed with the sword.
What makes this passage different from the previous two is that in these verses God is more active in this battle than in the previous two. When I say "more active", I mean that His work is more clearly seen and identified , in throwing the Amorites into confusion and then throwing down large hailstones from Heaven on them. God is always at work for His people and He never abandoned them to fight on their own, and they only fought in His strength; but in the previous two battles mentioned Israel violently pursued their enemies fully resting in the promise God had given beforehand. In this battle in Joshua 10, I'm sure combat was a lot easier for Israel's army when their enemies are suffering panic attacks from God in front of them while God later crushes their army with heavenly hailstones. In our fight with sin I think this is how God works as well.
There will be times where we will fight sin vigorously in God's strength, resisting temptation, anger, lust, pride, fear and so forth. We will have to wrestle and war violently with these sins through prayer, fasting, confession, scripture meditation and more, and this is all done while resting on the same promise that sin has already been defeated at the cross. But there will also be times where God will seem to be "more active" in our battles with sin. He may supernaturally take away evil desires, or supernaturally crush sinful habits, or make ways of deliverance in situations of temptation that only He can make. The Christian should be on lookout for God's working at all times in either of these ways , because ultimately He is at work in both of these sin fighting situations. We must always be at work as well.
Philippians 2:12-13 states
Therefore, my beloved, as you have always obeyed, so now, not only as in my presence but much more in my absence, work out your own salvation with fear and trembling, for it is God who works in you, both to will and to work for his good pleasure.
In our rigorous fighting against sin, God is at work. In our sudden abandoning and conquering of certain sins, God is at work. He is at work in our wills and in our work, and the work that He began at salvation in our hearts, He will bring to completion. Therefore we should rest in His promise that our greatest battle has been won already. Our sins and the principalities that chained us to them have been canceled and made an open show of. The professor has given His students an "A" at the beginning of the course and tells them to enjoy and learn the material. Jesus has defeated sin on the cross once and for all and all who believe and trust in His work, can find rest and rejuvenation in Him to beat the sin already defeated, and to take possession of the land already given to them.