In this connection Paul learnedly stresses the matter of time: that in this chapter Moses is speaking about righteousness and a righteous or justified Abraham prior to the Law, prior to the works of the Law, yes, prior to the people of the Law and before Moses, the lawgiver, was born. Accordingly, he says that righteousness is not only not from the Law but is prior to the Law, and that neither the Law nor the works of the Law contribute anything toward it.
Then what? Is the Law useless for righteousness? Yes, certainly. But does faith alone, without works, justify? Yes, certainly. Otherwise you must repudiate Moses, who declares that Abraham is righteous prior to the Law and prior to the works of the Law, not because he sacrificed his son, who had not yet been born, and not because he did this or that work, but because he believed God who gave a promise.
In this passage no mention is made of any preparation for grace, of any faith formed through works, or of any preceding disposition. This, however, is mentioned: that at that time Abraham was in the midst of sins, doubts, and fears, and was exceedingly troubled in spirit.
How, then, did he obtain righteousness? In this way: God speaks, and Abraham believes what God is saying. Moreover, the Holy Spirit comes as a trustworthy witness and declares that this very believing or this very faith is righteousness or is imputed by God Himself as righteousness and is regarded by Him as such.
Luther, M. (1999). Luther’s works, vol. 3: Lectures on Genesis: Chapters 15-20. (J. J. Pelikan, H. C. Oswald, & H. T. Lehmann, Eds.) (Vol. 3, pp. 20–21). Saint Louis: Concordia Publishing House.
