He did not want to give us His divinity unconcealed; this was impossible. For God said (Ex. 33:20): “Man shall not see Me and live.” Therefore it was necessary for God to hide, cover, and conceal Himself, thus enabling us to touch and apprehend Him. He must disguise Himself in flesh and blood, in the Word, in the external ministry, in Baptism, in the Sacrament and Lord’s Supper, where He gives us His body in the bread and His blood in the wine, to eat and to drink. He must conceal Himself in forms to which He adds His Word, in order that we may recognize Him. Then the Word is not an empty sound; the bread is not plain bread; the wine not plain wine; Baptism not simple, common water. To be sure, it remains 
water, bread, and wine, but not only that; it is now called His bread, His Word, His water, His flesh, and His blood. When the water is mixed with sugar, two elements are present, namely sugar and water. Similarly, here we find flesh and “My flesh,” blood and “My blood.”
Therefore Christ says; “The bread which I shall give you is My flesh.” What kind of flesh is meant? Not veal or beef found in cow barns. Of course, that is also flesh; but it is not “the flesh which I shall give for the life of the world.” This is a live and yet a dead flesh, a flesh that has died and by reason of its death makes the whole world alive. Not John the Baptist or Mary or any angel, but solely this Person, Christ, is involved. It is His flesh alone that does this. To it God wants to draw us and bind us. God is not to be sought or found outside the Person born of Mary, the Person endowed with real flesh and blood, and crucified. God is to be apprehended and found in the flesh and blood of Christ solely by faith. We must know that this flesh and blood, though real, not only have the qualities of flesh and blood but partake of the Divine, as I explained before with the illustration of the sugar water.
Take a different illustration [first used by John of Damascus]. Unheated iron is, of course, still iron. But when fire and heat are added, and it glows, I can say: This iron no longer has the qualities of iron; it is like fire. To be sure, it is iron, but it is diffused to such an extent with fire that when you see or touch it, you cannot call it iron but feel only the fire. It burns you; it is fire that you see with your eyes. And now if you want to burn a hole through a barrel with it, or brand something with it, it is not the iron that does this; it is the fire. If I were to take cold iron, iron not aglow with heat, I could never brand anything with it. This requires a red-hot iron. On the other hand, the fire will not accomplish these same things without, and apart from, the iron, where the fire burns and bores. Thus the divine power is present bodily in the humanity of Christ and does what God naturally does, or does what the fire in the iron does. Only flesh and blood are visible. But faith sees a Man, sees flesh and blood which is like fiery iron; for it is permeated with the Divine.
Luther’s Works, vol. 23, 123-124