You see that the promises of the Law concerning the temporal kingdom have a quality far different from the promises of grace or the spiritual promises. The promises of the Law are contingent upon the will and merits of men, as he says here: “If you are careful to keep.” Therefore they are not lasting or permanent; for no merits of men are permanent, just as the men themselves are not, regardless of how saintly they are. But the spiritual promises are contingent, not upon merits but solely upon the eternal and free mercy of God, “not upon man’s will or exertion but upon God’s mercy” (Rom. 9:16). Therefore in the area of the Law free will rules—or, rather, it perishes and destroys everything; but in the area of the Gospel grace and the Spirit rule and preserve.
Luther’s works, vol. 9: Lectures on Deuteronomy 1525