From the Word
15 And the angel of the LORD called to Abraham a second time from heaven, 16 and said, “By myself I have sworn, says the LORD, because you have done this, and have not withheld your son, your only son, 17 I will indeed bless you, and I will multiply your descendants as the stars of heaven and as the sand which is on the seashore. And your descendants shall possess the gate of their enemies, 18 and by your descendants shall all the nations of the earth bless themselves, because you have obeyed my voice.”
Genesis 22:15–18, RSV
The blessing here promised to the patriarch in his seed is simply the grace and salvation in Christ which the gospel presents to the whole world. For Christ is the seed of Abraham, his own flesh and blood, and in Christ all believing inquirers will be blessed. This promise to the patriarch was later more minutely set forth and more widely circulated by the prophets. All of them wrote of the advent of Christ, his grace and gospel. The divine promise was believed by the saints prior to the birth of Christ; thus, through the coming Messiah they were preserved and saved by faith.
But faith is not abolished in the fulfilment of the promise; rather it is established. As they in former time believed in the future fulfilment, we now believe in the completed fulfilment. Faith in the two instances is essentially the same; but one belief succeeds the other as fulfilment succeeds promise. In both cases faith is based on the seed of Abraham, that is, on Christ. In one instance it precedes his advent and in the other it follows. He who would now, like the Jews, believe in Christ yet to come, as if the promise were still unfulfilled, would be condemned. For he would make God a liar in holding that the word is unredeemed, contrary to fact. Were the promise not fulfilled, our salvation would be still far off; we would have to wait its future accomplishment.
Having in mind faith under these two conditions, we must now believe not only in the promise but in its past fulfilment. For though the faith of the fathers is one with our faith, they trusting in Christ to come and we in a Christ revealed, yet the gospel leads from the former faith to the latter. It is now necessary to believe the promise, and also its fulfilment. Abraham and the ancients were not called on to believe in the accomplished fulfilment, though they had the same Christ as we have. There is one faith, one spirit, one Christ, one community of saints; but they preceded, while we come after, Christ. Thus we—the fathers and ourselves—have had and still have a common faith in the one Christ, but under different conditions. Because of this common faith believers unite in Christ as one body.
Luther, Martin, and John Sander. Devotional Readings from Luther’s Works for Every Day of the Year. Augustana Book Concern, 1915, pp. 434–35.