Our observation of the 500th anniversary of Dr. Martin Luther’s publication of the Ninety-Five Theses took place a few months ago. However, this event only initiated the Lutheran Reformation. Luther’s rediscovery of the Gospel resulted from his study (and teaching of) the Holy Scriptures from 1515 to 1520. He developed the 
theology of justification by faith alone in Christ within the context of the debate over indulgences within the Western Church.
On March 28, 1518 (Palm Sunday) Martin Luther preached a sermon (later published in 1519) called A Sermon on Two Kinds of Righteousness. Originally recorded in Latin, Georg Spalatin also translated it into German. The traditional Epistle Reading for Palm Sunday provided the text for the sermon: “Have this mind among yourselves, which is yours in Christ Jesus, who, though he was in the form of God, did not count equality with God a thing to be grasped.” (Philippians 2:5-6).
In this sermon, Martin Luther explained his emerging understanding of the relationship between divine and human righteousness. First, he discussed the alien righteousness that God pours into the sinner from the outside. This is the righteousness, by which Christ justifies sinners. He grants this righteousness through baptism or at any time of true repentance. Christ shares everything with the believer as a bridegroom and bride share all things. Luther reiterated that Christ’s righteousness becomes our righteousness; even Christ Himself becomes ours through faith.
Christ’s righteousness, which He instills into believers by faith, is the foundation and source of all actual righteousness. It replaces and even does more than Adam’s original righteousness. Luther concluded:
Therefore, this alien righteousness, infused in us without our works by grace alone—while the Father, to be sure, inwardly draws us to Christ—is set opposite original sin, likewise alien, which we acquire without our works by birth alone. Christ daily drives out the old Adam more and more in accordance with the extent to which faith and knowledge of Christ grow. For [alien righteousness] is not infused entirely at once, but it begins, makes progress, and is finally perfected at the end through death.[1]
This passage served as a bridge from Luther’s description of alien righteousness to the second kind of righteousness. Notice also Luther’s use of the medieval theological term: infused (which was translated as ‘instilled’ in Luther’s Works edition.) At this point, Luther did not seem concerned with making the distinction between the infusion of righteousness and the imputation of righteousness. However, he did emphasize the external (alien) nature of Christ’s righteousness given to the believer.
Now Dr. Luther turned toward an explanation of the second kind of righteousness, which he called one’s proper righteousness. Believers accomplish this righteousness through working with Christ’s alien righteousness. This cooperation allows the Christian to accomplish truly good works through the mortification of the flesh and service to one’s neighbor with love. Luther equates the proper righteousness to the fruit of the spirit (Galatians 5:22). He explained:
This righteousness goes on to complete the first for it ever strives to do away with the old Adam and to destroy the body of sin. Therefore it hates itself and loves its neighbor; it does not seek its own good, but that of another, and in this its whole way of living consists. For in that it hates itself and does not seek its own, it crucifies the flesh. Because it seeks the good of another, it works love. Thus in each sphere it does God’s will, lives soberly with self, justly with the neighbor, devoutly toward God.[2]
Once Christ has granted his alien righteousness to the believer, he may imitate Christ with the proper righteousness. Since the soul does not need to seek its own righteousness, it may now look after the interests of others.
[1] Luther’s Works, vol. 31, pp. 297-99 (quote on p. 299); I have slightly changed the translation based on the Latin text in Weimar Ausgabe, vol. 2, p. 146.
[2] LW 31: 300.