Lectures on Genesis Ch 15:6 (ca 1535-1540)

6. And he believed the Lord; and He reckoned it to him as righteousness.

No one has treated this passage better, more richly, more clearly, and more powerfully than St. Paul in the third to the twelfth chapters of Romans. Moreover, Paul treats it in such a way as to show that this promise concerning Abraham’s descendants should not be interpreted to apply solely to the legitimate physics or temporal seed but to the spiritual and eternal heritage.

For Moses uses heavenly things to illustrate his point, not things that are earthly or temporal. Therefore the promise, too, is a heavenly one, not of children of the flesh but of the spirit, or, as Paul calls them, “children of the promise” (Rom. 9:8). This meaning is clear from what Paul says.

Moreover, when Moses adds that Abraham believed God, this is the first passage of Scripture which we have had until now about faith. For the others, which Moses mentioned previously—the passage about the Seed of the woman, for example, the command to build the ark, the threat of the Flood, and the command to Abraham to leave his country, etc.—merely demand faith; they do not praise or recommend it.

These promises as well as the threats are all words which require faith, but they do not commend faith as the passage before us does. Therefore this is one of the foremost passages of all Scripture.
And Paul has not only expounded this passage most carefully; he also takes great pains to commend it to the church when he adds this statement (Rom. 4:23): “But the words ‘it was reckoned to him’ were written not for his [Abraham’s] sake alone”—who later on died—but (Rom. 15:4) “for our instruction, that … we might have hope.”

This is truly an instance of treating the Scriptures in an apostolic manner and of establishing the universal statement which is so dreadful and detestable to the very gates of hell: that all who believe the Word of God are just.

Accordingly, lest my discussions obscure what the best interpreter says, I shall speak rather briefly here. Read Paul, and read him most attentively. Then you will see that from this passage he constructs the foremost article of our faith—the article that is intolerable to the world and to Satan—namely, that faith alone justifies, but that faith consists in giving assent to the promises of God and concluding that they are true.

Luther, M. (1999, c1961). Vol. 3: Luther's works, vol. 3 : Lectures on Genesis: Chapters 15-20 (J. J. Pelikan, H. C. Oswald & H. T. Lehmann, Ed.). Luther's Works (Ge 15:6). Saint Louis: Concordia Publishing House.