Martin Luther's Writings

You must get used to the idea that it is one thing to do the works of the law and quite another to fulfill it. The works of the law are every thing that a person does or can do of his own free will and by his own powers to obey the law. But because in doing such works the heart abhors the law and yet is forced to obey it, the works are a total loss and are completely useless. That is what St. Paul means in chapter 3 when he says, "No human being is justified before God through the works of the law." From this you can see that the schoolmasters [i.e., the scholastic theologians] and sophists are seducers when they teach that you can prepare yourself for grace by means of works. How can anybody prepare himself for good by means of works if he does no good work except with aversion and constraint in his heart? How can such a work please God, if it proceeds from an averse and unwilling heart?

But to fulfill the law means to do its work eagerly, lovingly and freely, without the constraint of the law; it means to live well and in a manner pleasing to God, as though there were no law or punishment. It is the Holy Spirit, however, who puts such eagerness of unconstained love into the heart, as Paul says in chapter 5. But the Spirit is given only in, with, and through faith in Jesus Christ, as Paul says in his introduction. So, too, faith comes only through the word of God, the Gospel, that preaches Christ: how he is both Son of God and man, how he died and rose for our sake. Paul says all this in chapters 3, 4 and 10.

That is why faith alone makes someone just and fulfills the law; faith it is that brings the Holy Spirit through the merits of Christ. The Spirit, in turn, renders the heart glad and free, as the law demands. Then good works proceed from faith itself. That is what Paul means in chapter 3 when, after he has thrown out the works of the law, he sounds as though the wants to abolish the law by faith. No, he says, we uphold the law through faith, i.e. we fulfill it through faith.

Sin in the Scriptures means not only external works of the body but also all those movements within us which bestir themselves and move us to do the external works, namely, the depth of the heart with all its powers. Therefore the word "do" should refer to a person's completely falling into sin. No external work of sin happens, after all, unless a person commit himself to it completely, body and soul. In particular, the Scriptures see into the heart, to the root and main source of all sin: unbelief in the depth of the heart. Thus, even as faith alone makes just and brings the Spirit and the desire to do good external works, so it is only unbelief which sins and exalts the flesh and brings desire to do evil external works. That's what happened to Adam and Eve in Paradise (cf. Genesis 3).