Paul then, in Romans 3-5, declares the only remedy available: the righteousness that comes to sinners freely by grace through faith in Jesus Christ, in other words - the gospel: the Righteousness that comes to sinners not through any works, but solely through trusting in what God has done in Christ for sinners. Looking then at the two places in Romans 6 where Paul then asks that well known rhetorical question, how does he respond to the charge that too much of this abundant grace will lead to antinomianism or licentiousness? What does Paul offer that will keep believers from falling back into sinful living now that they are freely forgiven? After all, isn't this grace, more or less, a get-out-of-jail-free card? Many would answer now that we have been justified by faith it's necessary that the law comes back in as that which shepherds and keeps believers from sinning - keeps them on the straight and narrow. Grace in the dock: does abundant grace lead to increased sin?
Question: What shall we say then? Shall we continue in sin, that grace may abound?
Answer: God forbid. We who died to sin, how shall we any longer live therein? Or are ye ignorant that all we who were baptized into Christ Jesus were baptized into his death? We were buried therefore with him through baptism unto death: that like as Christ was raised from the dead through the glory of the Father, so we also might walk in newness of life. For if we have become united with him in the likeness of his death, we shall be also in the likeness of his resurrection; knowing this, that our old man was crucified with him, that the body of sin might be done away, that so we should no longer be in bondage to sin; for he that hath died is justified from sin. But if we died with Christ, we believe that we shall also live with him; knowing that Christ being raised from the dead dieth no more; death no more hath dominion over him. For the death that he died, he died unto sin once: but the life that he liveth, he liveth unto God. Even so reckon ye also yourselves to be dead unto sin, but alive unto God in Christ Jesus. Let not sin therefore reign in your mortal body, that ye should obey the lusts thereof: neither present your members unto sin as instruments of unrighteousness; but present yourselves unto God, as alive from the dead, and your members as instruments of righteousness unto God. For sin shall not have dominion over you: for ye are not under law, but under grace. (Rom. 6:1-14)
Question: What shall we say then? Shall we continue in sin, that grace may abound?
Answer: God forbid. Know ye not, that to whom ye present yourselves as servants unto obedience, his servants ye are whom ye obey; whether of sin unto death, or of obedience unto righteousness? But thanks be to God, that, whereas ye were servants of sin, ye became obedient from the heart to that form of teaching whereunto ye were delivered; and being made free from sin, ye became servants of righteousness. I speak after the manner of men because of the infirmity of your flesh: for as ye presented your members as servants to uncleanness and to iniquity unto iniquity, even so now present your members as servants to righteousness unto sanctification. For when ye were servants of sin, ye were free in regard of righteousness. What fruit then had ye at that time in the things whereof ye are now ashamed? for the end of those things is death. But now being made free from sin and become servants to God, ye have your fruit unto sanctification, and the end eternal life. (Rom. 6:15-22)
Paul counters not with law, but grace, i.e. more truth of what God has done in Christ to save sinners from the penalty and power of sin. In other words Paul gives more gospel to the believer which is the power of God unto salvation (Rom 1:16). The law, be it first or third use, has no power to draw one away from sin or overcome it. Law can only tell us our duty and expose how we in fact do the things we ought not and don't do the things we ought. When tempted to sin, the problem is not our lack of knowledge of the law (don't do that!... do this!). Indeed, at those times the law offers no aid or power to resist sin's dominion because that is not the purpose given it by God. Rather our problem is a lack of faith and trust in Christ's finished work on our behalf. And more faith comes only by hearing more gospel (Rom. 10:17). The transforming power of the Spirit is communicated to the believer not by the hearing and doing of the law but by hearing and believing(!) the good news of what God has done for us in Christ. The works of God come by believing (John 6:28-29).
Our problem as sinners isn't a knowledge problem remedied by more law instruction and our subsequent doing, but a sin-problem which only the gospel solves as we trust in God's Already Done in Christ! This is true for our justification as well as our sanctification. As believers we are called to mortify sin within us, which by definition means resisting the desires that actually tempt us and then again to offer ourselves to loving God and loving our neighbor. The finished work of Jesus declared in the gospel is the only weapon given believers that actually breaks that power of sin and frees us to walk in a righteous direction.
10 For in that he died, he died unto sin once: but in that he liveth, he liveth unto God. 11Likewise reckon ye also yourselves to be dead indeed unto sin, but alive unto God through Jesus Christ our Lord.
It is the blood of our Lord Jesus Christ applied by the Holy Spirit through the gospel proclaimed in Word and Sacrament that cleanses believing sinners from sin and sets them apart unto holiness (all things are sanctified by blood - Hebrews 9:22). The third use of the law?? With our eyes of faith upon Christ alone, the Holy Spirit uses the law now written on our hearts to direct and inform us in our sanctification. Our reasonable offering to God is obedience to Him in the bright light and power of what our Lord Jesus Christ has done for us. As forgiven sinners we indeed need to hear the law in order to better capture our attention as to what it really means to live a holy life, so as to not water-down sin nor lower the high perfection of God's righteousness to which we are called. But the power of the Spirit unto holiness is apprehended only through faith in the finished work of Christ Jesus.
Now, it's certainly the case that there are perversions of God's grace and distortions of the gospel which can lead someone to become indifferent to his sin. I imagine this is especially so where Law and Gospel are not proclaimed together. But the solution to that problem is not a greater emphasis on the law in order to correct or balance out that distortion. Rather, the solution is to clearly preach and teach the law's diagnosis and judgment of sinners and God's divinely appointed remedy offered in the gospel. And that gospel declares that we who have been buried with Christ have died to sin. Do we believe that? Do we trust that Christ has accomplished that on our behalf? If so, then how can we who have died to sin be indifferent to it? If we continue to live a life under sin's rule, isn't it because we're not trusting in what God has done, i.e. not believing? If I do believe that I have died to sin in Christ and am no longer under law but under grace, then it follows that I will offer myself to live in a manner consistent with that faith and repentance, however imperfect; seeking to live in accord with righteousness - not merely because the law tells me I should (which it certainly does), but because the grace of God (which I am now under) has released me from the dominion of sin and renewed me with a new heart and new will which desires to live in obedience to Him.
Do we still sin? Of course. And so it seems to me that the piety of the church should be known more by a faith in Christ and repentance of sin than an absence of sin. The grace of God in Christ both fulfills the law perfectly for us in our justification and delivers us completely from sin's dominion which leads us to the doing of good works acceptable to God through faith. The law points the way we should live and informs us when we don't. The grace of God unites us to Christ, pardons our sin, delivers us from sin's dominion, justifies us before God with a righteousness not our own, gives us a new heart and will born of God, and renews us daily unto obedience through repentance and faith in Christ.