"Even reason teaches that your lives must conform to your business; each is in duty bound to obey him whom he serves. As Christians you are obliged to render another service than that you gave when under the dominion of sin, and obedient to it; when you were unable to escape its power and to do any work good before God. You have now come out of bondage and are relieved from obedience to sin, through grace, having devoted yourselves to the service of God, to obeying him. Therefore, assuredly you must change your manner of life."
12. Truly, Paul here argues reasonably and within the scope of man's natural understanding. We preach the same truths, but, presenting them in the form of Christian doctrine, we necessarily employ different language and a loftier tone, lest it be offensive to the world. We may say that theft, murder, envy, hate and other crimes and vices are transgressions, yet we cannot remedy the evils by the mere prohibitions of the law. The remedy must be effected through God's grace, and is accomplished in the believer, not by our power, but by the Holy Spirit. But when we so explain, the stupid world immediately blurts out, "Oh, if it be true that our works do not remedy evils, let us enjoy ourselves and not bother about good works!"
13. That their implication is false and a wanton perversion of the true doctrine is manifest from the fact that we exalt and endorse the command of God, and also the doctrine of reason, that teach us to do good and avoid evil. Indeed, we assist reason, which is powerless to remedy evil. If reason were itself sufficient, men would not permit themselves to be deceived by their own visionary ideas and false doctrines about worthless and vain works, as are followers of the papacy and of all false worship. No doubt such error has its rise in the principle that we are to do good and avoid evil. The principle fundamentally is true, and accepted by all men; but when it comes to the theories we build upon it, the speculations as to how it is to be put into practice, there is disagreement. Only the Word of God can show how to accomplish it.
Reason is easily blinded on this point and deceived by false appearances, being led by anything merely called good. Even when it has performed all it believes to be right, it is still uncertain of acceptance. Indeed, it perceives no fruits, no benefit, to result from its teaching; for at best its achievements extend no farther than outward works—the object being to make the doer appear righteous and respectable before men—while inward sinfulness is unrestrained and the soul remains captive to its former life, obedient to the lusts of sin. And the motive of such a one is not sincere; he would conduct himself quite otherwise were he not restrained by fear of shame and punishment.