A. When Our Lord said, "This is My body," the substance of the bread was changed into the substance of His body. When He said, "This is My blood," the substance of the wine was changed into the substance of His blood.
"Substance" literally means that which stands underneath. Underneath what? Underneath the outward appearances or qualities—such as color, taste, figure, smell, etc.—that are perceptible to our senses. Therefore we never see the substance of anything. Of this seat, for instance, I see the color, size, and shape; I feel the hardness, etc.; but I do not see the substance, namely, the wood of which it is made. When the substance of anything is changed, the outward appearances change with it. But not so in the Holy Eucharist; for by a miracle the appearances of bread and wine remain the same after the substance has been changed as they were before. As the substance alone is changed in the Holy Eucharist, and as I cannot see the substance, I cannot see the change. I am absolutely certain, however, that the change takes place, because Our Lord said so; and I believe Him, because He could not deceive me. He is God, and God could not tell a lie, because He is infinite truth. This change is a great miracle, and that is the reason we cannot understand it, though we believe it. Once at a marriage in Cana of Galilee (John 2) Our Lord changed water into wine. The people were poor, and Our Lord, His Blessed Mother, and the Apostles were present at the wedding when the wine ran short; and our Blessed Lady, always so kind to everyone, wishing to spare these poor people from being shamed before their friends, asked Our Lord to perform the miracle, and at her request He did so, and changed many vessels of water into the best of wine. In that miracle Our Lord changed the substance of the water into the substance of the wine. Why, then, could He not change in the same way and by the same power the substance of bread and wine into the substance of His own body and blood? When He changed the water into wine, besides changing the substance, He changed everything else about it; so that it had no longer the appearance of water, but everyone could see that it was wine. But in changing the bread and wine into His body and blood He changes only the substance, and leaves everything else unchanged so that it still looks and tastes like bread and wine; even after the change has taken place and you could not tell by looking at it that it was changed. You know it only from your faith in the words of our divine Lord, when He tells you it is changed.
Again, it is much easier to change one thing into another than to make it entirely out of nothing. Anyone who can create out of nothing can surely change one thing into another. Now Our Lord, being God, created the world out of nothing; and He could therefore easily change the substance of bread into the substance of flesh. I have said Our Lord's body in the Holy Eucharist is a living body, and every living body contains blood; and that is why we receive both the body and the blood of Our Lord under the appearance of the bread alone. The priest receives the body and blood of Our Lord under the appearance of both bread and wine, while the people receive it only under the appearance of bread. The early Christians used to receive it as the priest does—under the appearance of bread and under the appearance of wine; but the Church had to make a change on account of circumstances. First, all the people had to drink from the same chalice or cup, and some would not like that, and show disrespect for the Blessed Sacrament by refusing it. Then there was great danger of spilling the precious blood, passing it from one to another; and finally, some said that Christ's blood was not in His body under the appearance of bread. This was false; and to show that it was false, and for the other reasons, the Church after that gave Holy Communion to the people under the appearance of bread alone. The Church always believes and teaches the same truths. It always believed that the Holy Eucharist under the appearance of bread contained also Our Lord's blood; but it taught it more clearly when it was denied.