Sin will not rule as lord over us while we are "under grace" (Romans 6:14), God is at work in us to will and to do his good pleasure. I based this on Romans 6:17, from the context which says, "But thanks be to God that though you were slaves of sin, you became obedient from the heart to that form of teaching to which you were committed." Since Paul thanks God that the Romans became obedient from the heart, I conclude that God is the one who worked to bring about this obedience in their hearts and if God works to bring about obedience in our heart, then sin will not be the lord over us, God will.
Let me reinforce a caution at this point. This does not mean we become perfect in this life (Philippians 3:12; 2 Corinthians 3:18; Romans 7:24), but that it means sin is dethroned and the destruction of sin is certain as we "fight the good fight of faith" (1 Timothy 6:12) until we die or Jesus comes (2 Timothy 4:7).
I implore you not to turn the sovereignty of God into a permission for passivity, but a reason to hope. Let the sovereignty of God make you hopeful that change is possible, not passive as if no change were necessary. Now let's take the following texts as encouragements from God that you can and youwill make progress in driving sin from your life.
2 Thessalonians 1:11-12, "We pray for you always, that our God will . . . fulfill every desire for goodness and the work of faith with power, so that the name of our Lord Jesus will be glorified in you." Christ gets the glory when it is manifest that God enables us to fulfill our good resolves through him.
Hebrews 13:20-21, "Now the God of peace . . . equip you in every good thing to do His will, working in us that which is pleasing in His sight, through Jesus Christ, to whom be the glory forever and ever. Amen." Since it is God who enables us to do what is pleasing in his sight "through Jesus," it is Jesus who gets the glory, not us.
1 Peter 4:11, "Whoever . . . serves is to do so as one who is serving by the strength which God supplies; so that in all things God may be glorified through Jesus Christ, to whom belongs the glory and dominion forever and ever. Amen." The giver gets the glory because God is the one who enables us to "serve" him, he gets the credit for the service.
Galatians 5:22-23, "The fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control; against such things there is no law." Christian attitudes and behaviors are the fruit of the Spirit, not ultimately the fruit of our own efforts. Our efforts are essential, but not finally decisive. See below on Philippians 2:12-13.
These texts are examples of the fulfillment of the Old Testament promise of the New Covenant in which God works in his people to bring about obedience. Here are some examples of those Old Testament promises.
Jeremiah 31:31-33, "Behold, days are coming, declares the LORD, when I will make a new covenant with the house of Israel. . . . I will put My law within them and on their heart I will write it." Once, the law was external on stone and met rebellion in our rebellious hearts. However, in the New Covenant God does not leave the law outside, making demands; he also takes it inside, creating obedience.
Deuteronomy 30:6, "The LORD your God will circumcise your heart . . . to love the LORD your God with all your heart and with all your soul."
Ezekiel 11:19-20, "I will . . . put a new spirit within them. And I will take the heart of stone out of their flesh and give them a heart of flesh, that they may walk in My statutes and keep My ordinances and do them."
Ezekiel 36:26-27, "I will give you a new heart and put a new spirit within you; and I will remove the heart of stone from your flesh and give you a heart of flesh. I will put My Spirit within you and cause you to walk in My statutes." Note the strong language of "cause you to walk in my statutes." That is what I think Paul was thanking God for in Romans 6:17.
Jeremiah 32:40, "I will make an everlasting covenant with them that I will not turn away from them, to do them good; and I will put the fear of Me in their hearts so that they will not turn away from Me." Our enduring to the end in the fear of God is owing to God's powerful grace to keep us.
How then should we pray? One example from Paul: "May the Lord cause you to increase and abound in love for one another, and for all people" (1 Thessalonians 3:12; see Philippians 1:9-11).
Should we use our willpower to obey? Yes. Mightily. "Work out your salvation with fear and trembling; for it is God who is at work in you, both to will and to work for His good pleasure"(Philippians 2:12-13).