2 Then Paul presents again the question raised by the obstinate world when it encounters this doctrine. V.15. “What then?” he asks, “shall we sin because we are not under the law but under grace ?” It is the perversity of the world that, when we preach about forgiveness of sins by pure grace and without merit of man, it should either say we forbid good works, or else try to draw the conclusion that man may continue to live in sin and follow his own pleasure; when the fact is, we should particularly strive to live a life the very reverse of sinful, that our doctrine may draw people to good works, unto the praise and honor and glory of God. Our doctrine, rightly apprehended, does not influence to pride and vice, but to humility and obedience. 3 In affairs of temporal government, whether domestic or civil, judge or ruler, it is understood that he who asks for pardon confesses himself guilty, acknowledges his error and promises to reform — to transgress no more. For instance, when the judge extends mercy and pardon to the thief deserving of the gallows, the law is canceled by grace. Suppose now the thief continues in wrong-doing and boasts, “Now that I am under grace I may do as I please, I have no law to fear”; who would tolerate him? For though the law is indeed canceled for him and he receives not merited punishment, though grace delivers him from the rope and the sword, life is not granted him that he may continue to steal, to murder; rather he is supposed to become honest and virtuous. If he does not, the law will again overtake him and punish him as he deserves. In short, where grace fulfills the law, no one is for that reason given license to continue in wrong-doing; on the contrary, he is under increased obligation to avoid occasions of falling under condemnation of the law. 4 Everyone can readily comprehend this principle in temporal things; no one is stupid enough to tolerate the idea of grace being granted to extend opportunity to do wrong. It is only the Gospel doctrine concerning God’s grace and the forgiveness of sin that must suffer the slanderous misrepresentation that makes it abolish good works or give occasion for sin. We are told how God, in his unfathomable grace, has canceled the sentence of eternal death and hell fire which, according to the Law and divine judgment, we deserved, and has given us instead the freedom of life eternal; thus our life is purely of grace. Yet certainly we are not pardoned that we may live as before when, under condemnation and wrath, we incurred death. Rather, forgiveness is bestowed that we in appreciation of the sublimity and sanctity of God’s unspeakably great blessing which delivers us from death unto life, should henceforth take heed that we lose it not; that we fall not from grace to pass again under judgment and the sentence of eternal death. We are to conduct ourselves as men made alive and saved. 5 So Paul says in V.16. “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?” Meaning, Since you now have, under grace, obtained forgiveness of sin and are become righteous, you owe it to God to live in obedience to his will. Necessarily your life must be obedient to some master. Either you obey sin, to continue in the service of which brings death and God’s wrath, or you obey God, in grace, unto a new manner of life. So, then, you are no more to obey sin, having been freed from its dominion and power.
–Luther on Romans 6