One of the prominent ways in which Luther spoke of the imputation of Christ’s righteousness was through the double exchange of the believer’s sin to the sinless Christ, and Christ’s righteousness to the sinful believer:
“He has made His righteousness my righteousness, and my sin His sin. If He has made my sin to be His sin, then I do not have it, and I am free. If He has made His righteousness my righteousness, then I am righteous now with the same righteousness as He” (25:188).
It was this precious truth that gave Luther peace of conscience knowing that his sin would not be counted against him because it was borne by another, and that his justification depended not upon his own works, but on the merits of Christ. Timothy George summarizes Luther’s teaching on the exchange that takes place between Christ and the Christian: “God accepts the righteousness of Christ, which is alien to our nature proper, as our own. Though our sins are not actually removed, they cease to be counted against us.” Diarmaid MacCulloch similarly defines Luther’s understanding of justification in this way: “God ‘imputes’ the merits of the crucified and risen Christ through grace to a fallen human being, who remains without inherent merit and who, without this ‘imputation’, would remain unrighteous.”
Christ took upon himself our sin, bore God’s wrath in our place, clothes us in his innocence, and by this sweet exchange frees us from the curse of the law. Luther taught that in justification our sins are not imputed to us but forgiven. Our sins instead belong to Christ and his perfect righteousness is now our own. Luther beautifully expresses this truth in one of his letters:
“Therefore, my dear Friar, learn Christ and him crucified. Learn to praise him and, despairing of yourself, say, ‘Lord Jesus, you are my righteousness, just as I am your sin. You have taken upon yourself what is mine and have given to me what is yours. You have taken upon yourself what you were not and have given to me what I was not'” (48:12).
Before Christ’s righteousness can be grasped by the empty hand of faith, the sinner must despise his own righteousness and good works. For Luther, the purpose of the law is to show the sinner the impossibility of gaining salvation by his or her own merits when confronted with the utter holiness of God’s perfect righteousness. The only way to be righteous before God is to cast oneself upon the righteousness of Christ who alone fulfilled the law’s requirements in our place. The only righteousness which God will accept on the day of judgment is his own which comes through faith in Christ who is God himself. Luther describes in detail this glorious exchange which takes place at justification:
“Is not this a beautiful, glorious exchange, by which Christ, who is wholly innocent and holy, not only takes upon himself another’s sin, that is, my sin and guilt, but also clothes and adorns me, who am nothing but sin, with his own innocence and purity? And then besides dies the shameful death of the Cross for the sake of my sins, through which I have deserved death and condemnation, and grants to me his righteousness, in order that I may live with him eternally in glorious and unspeakable joy. Through this blessed exchange, in which Christ changes places with us (something the heart can grasp only in faith), and through nothing else, are we freed from sin and death and given his righteousness and life as our own” (51:315).