“The second promise of Christ was to Abraham, Genesis 22[:18], where God said, “In your seed shall all the Gentiles be blessed.” If all the Gentiles are to be blessed, then it is certain that otherwise, apart from this seed of Abraham, they were all unblessed and under a curse. From this it follows that human nature has nothing but cursed seed and bears nothing but unblessed fruit; otherwise, there would be no need for all of them to be blessed through this seed of Abraham. Whoever says “all” excludes no one; therefore, apart from Christ, all who are born of man must be under the devil, cursed in sin and death.
Here again the mother of God is proven to be a pure virgin. For since God cannot lie, it was inevitable that Christ should be the seed of Abraham, that is, his natural flesh and blood, like all of Abraham’s descendants. On the other hand, because he was to be the blessed seed which should bless all others, he could not be begotten by man, since such children, as has been said, cannot not be conceived without sin because of the corrupt and tainted flesh, which cannot perform its function without taint and sin.
Thus the word, by which God promises that Christ will be the seed of Abraham, requires that Christ be born of a woman and be her natural child. He does not come from the earth like Adam [Gen. 2:7]; neither is he from Adam’s rib like Eve [Gen. 2:21–22]. He comes rather like any woman’s child, from her seed. The earth was not the natural seed for Adam’s body; neither was Adam’s rib the natural seed for Eve’s body. But the virgin’s flesh and blood, from which children come in the case of all other women, was the natural seed of Christ’s body. And she too was of the seed of Abraham” (Luther’s Works, v.45 p.203-204).