Luther on Genesis 22:18 “And by your Seed shall all the nations of the earth be blessed, because you have obeyed My voice.”
Consequently, the cause is pointed out in this promise, which bears witness that all nations are under the curse and power of the devil and nevertheless offers deliverance if they recognize their wretched state and do not despair but believe in the Seed and bless themselves because of this Seed, boast of life, and individually apply this deliverance to themselves by faith and say: “I am no longer a sinner; I am righteous. I am not cursed; I am blessed through the Seed of Abraham—the Seed who is true man, born of the descendants of Abraham, and true God.”
This blessing is so powerful and efficacious that it is able to destroy and abolish both death and the entire curse which was brought on as a result of original sin.
Moreover, the fact that God, as is stated in the Epistle to the Hebrews (2:16), is concerned with the descendants of Abraham, not with angels, is an incalculably great honor to that wretched mass of the human race. For it was not difficult or impossible for Him to bring His Son into the world without a mother. But He wanted to make use of the female sex.
He could likewise have formed a body suddenly from a virgin, just as He formed Adam from clay and Eve from a rib of Adam. He did not choose to do this, but He adhered to the order which He Himself had established. For a maiden has been created in such a way that she should conceive, be with child for nine months, and give birth. Therefore He wanted His Son to be conceived, carried and born in the womb of a maiden, not formed from clay and not conceived by a male.
It is surely a great comfort that it did not please God that His Son should become man from any other material than the human race. He wanted His Son to become our brother and to adorn us with the exceedingly great honor of having a God born and made man in our flesh and blood.
These facts are so grand and sublime that they cannot be apprehended and understood in any other manner than by faith, which produces in us that spiritual confidence through which we firmly believe that we have eternal peace in heaven and on earth—not in ourselves, of course, but in the Seed of Abraham.
Luther’s works, vol. 4: Lectures on Genesis: Chapters 21-25 p 161-162