“Christ is promised for the first time soon after Adam’s fall, when God said to the serpent, “I will put enmity between you and the woman, and between your seed and her seed; he shall crush your head, and you shall bruise his heel” [Gen. 3:15]. Here I defer demonstrating that the serpent spoke possessed of the devil, for no dumb beast is so clever that it can utter or comprehend human speech, much less speak or inquire about such exalted matters as the commandment of God, as the serpent does here. Therefore, it must certainly have been a rational, highly intelligent, and mighty spirit which was able to utter human speech, deal so masterfully with God’s commandments, and seize and employ human reason.
Since it is certain that a spirit is something higher than a man, it is also certain that this is an evil spirit and an enemy of God, for it breaks God’s commandment and acts contrary to his will. Therefore, it is undoubtedly the devil. And so the word of God which speaks of crushing the head must refer also to the devil’s head; though not to the exclusion of the natural head of the serpent, for with a single word he speaks of both devil and serpent as of one thing. Therefore, he means both heads. But the devil’s head is that power by which the devil rules, that is, sin and death, by means of which he has brought Adam and all Adam’s descendants under his control” (Luther’s Works, v.45 p. 201-202).