The Word of God cannot be received and cherished by any works whatever but only by faith. Therefore it is clear that, as the soul needs only the Word of God for its life and righteousness, so it is justified by faith alone and not any works; for if it could be justified by anything else, it would not need the Word, and consequently it would not need faith.

This faith cannot exist in connection with works—that is to say, if you at the same time claim to be justified by works, whatever their character—for that would be the same as “limping with two different opinions” [I Kings 18:21], as worshiping Baal and kissing one’s own hand [Job 31:27–28], which, as Job says, is a very great iniquity. Therefore the moment you begin to have faith you learn that all things in you are altogether blameworthy, sinful, and damnable, as the Apostle says in Rom. 3[:23], “Since all have sinned and fall short of the glory of God,” and, “None is righteous, no, not one; … all have turned aside, together they have gone wrong” (Rom. 3:10–12). When you have learned this you will know that you need Christ, who suffered and rose again for you so that, if you believe in him, you may through this faith become a new man in so far as your sins are forgiven and you are justified by the merits of another, namely, of Christ alone.

Since, therefore, this faith can rule only in the inner man, as Rom. 10[:10] says, “For man believes with his heart and so is justified,” and since faith alone justifies, it is clear that the inner man cannot be justified, freed, or saved by any outer work or action at all, and that these works, whatever their character, have nothing to do with this inner man. On the other hand, only ungodliness and unbelief of heart, and no outer work, make him guilty and a damnable servant of sin. Wherefore it ought to be the first concern of every Christian to lay aside all confidence in works and increasingly to strengthen faith alone and through faith to grow in the knowledge, not of works, but of Christ Jesus, who suffered and rose for him, as Peter teaches in the last chapter of his first Epistle (I Pet. 5:10). No other work makes a Christian. Thus when the Jews asked Christ, as related in John 6[:28], what they must do “to be doing the work of God,” he brushed aside the multitude of works which he saw they did in great profusion and suggested one work, saying, “This is the work of God, that you believe in him whom he has sent” [John 6:29].

—Martin Luther
“Freedom of a Christian,” Works, 31:346–347.