And taking bread, he gave thanks, and brake; and gave to them, saying:
"This is my body, which is given for you. Do this for a commemoration of me."
“As they were eating he took bread and broke it.” Christ instituted this sacrament at the time of the passover in order to teach us by every possible means both that he himself had been the lawgiver of the Old Testament, and also that the whole of the Old Testament had been a foreshadowing of these mysteries. He was replacing the type by the reality.
The fact that it was evening signified that the fullness of time had come and that all was about to be accomplished. He gave thanks to teach us how we ought to celebrate these mysteries, to show that he was not going to his passion against his will, and to train us to accept with gratitude whatever we have to suffer and so to derive from it hope of future blessedness.
If the type was able to free a people from bondage, much more would the reality liberate the world, and Christ’s death bring down blessings upon our race. We see then why he did not institute this sacrament before, but only when it was time to abolish the rites of the law.
Christ put an end to the most important Jewish festival by offering his disciples another far more awe-inspiring meal. “Take, eat,” he said, “this is my body which is broken for many.” He told them that the reason he was going to suffer was to take away our sins.
He spoke of the blood of the new covenant, that is of the promise, the new law. He had promised long before that the new covenant would be ratified by his blood. As the old covenant had been ratified by the blood of sheep and calves, so the new covenant was to be ratified by the blood of the Lord.
Thus, by speaking of his covenant and by reminding them that the old covenant had also been inaugurated by the shedding of blood, he made known to them that he was soon to die. And he told them once again the reason for his death in the words, “This is my blood, which is poured out for all for the forgiveness of sins and, Do this in memory of me.”
Notice how he leads them away from the Jewish customs by saying, “Just as you used to do this in memory of the miracles performed in Egypt, so now you must do it in memory of me.”
Blood was shed then for the salvation of the firstborn: It is to be shed now for the forgiveness of the sins of the whole world. “This,” he said, “is my blood, which is shed for the forgiveness of sins.” He said this to show that his passion and cross are a mystery, and so again to comfort his disciples.
As Moses had said, “This shall be for you an everlasting memorial,” so now the Lord says, “Do this in memory of me until I come.” This is why he also says, “I have longed to eat this passover,” meaning, “I have longed to hand over to you these new rites, and to give you the passover which will turn you into people moved by the Spirit.”
~St. John Chrysostom, c. 347-407
Full and clear ring out your chanting,
Joy nor sweetest grace be wanting,
From your heart let praises burst:
For today the feast is holden,
When the institution olden
Of that supper was rehearsed.
Here the new law’s new oblation,
By the new king’s revelation,
Ends the form of ancient rite:
Now the new the old effaces,
Truth away the shadow chases,
Light dispels the gloom of night.
What he did at supper seated,
Christ ordained to be repeated,
His memorial ne’er to cease:
And his rule for guidance taking,
Bread and wine we hallow, making
Thus our sacrifice of peace.
~from "Lauda Sion" by St. Thomas Aquinas