The Passover
3 Tell all the congregation of Israel that on the tenth day of this month every man shall take a lamb according to their fathers' houses, a lamb for a household. 4 And if the household is too small for a lamb, then he and his nearest neighbor shall take according to the number of persons; according to what each can eat you shall make your count for the lamb. 5 Your lamb shall be without blemish, a male a year old. You may take it from the sheep or from the goats, 6 and you shall keep it until the fourteenth day of this month, when the whole assembly of the congregation of Israel shall kill their lambs at twilight.
7 “Then they shall take some of the blood and put it on the two doorposts and the lintel of the houses in which they eat it. 11 It is the Lord's Passover. 12 For I will pass through the land of Egypt that night, and I will strike all the firstborn in the land of Egypt, both man and beast; and on all the gods of Egypt I will execute judgments: I am the Lord. 13 The blood shall be a sign for you, on the houses where you are. And when I see the blood, I will pass over you, and no plague will befall you to destroy you, when I strike the land of Egypt.
21 Then Moses called all the elders of Israel and said to them, “Go and select lambs for yourselves according to your clans, and kill the Passover lamb. 22 Take a bunch of hyssop and dip it in the blood that is in the basin, and touch the lintel and the two doorposts with the blood that is in the basin. None of you shall go out of the door of his house until the morning. 23 For the Lord will pass through to strike the Egyptians, and when he sees the blood on the lintel and on the two doorposts, the Lord will pass over the door and will not allow the destroyer to enter your houses to strike you (Exod 12:3-7,11-13,21-22).
The Last Supper
7 Then came the day of Unleavened Bread, on which the Passover lamb had to be sacrificed. 8 So Jesus sent Peter and John, saying, “Go and prepare the Passover for us, that we may eat it.”
14 And when the hour came, he reclined at table, and the apostles with him. 15 And he said to them, “I have earnestly desired to eat this Passover with you before I suffer. 16 For I tell you I will not eat it until it is fulfilled in the kingdom of God.” 17 And he took a cup, and when he had given thanks he said, “Take this, and divide it among yourselves. 18 For I tell you that from now on I will not drink of the fruit of the vine until the kingdom of God comes.” 19 And he took bread, and when he had given thanks, he broke it and gave it to them, saying, “This is my body, which is given for you. Do this in remembrance of me.” 20 And likewise the cup after they had eaten, saying, “This cup that is poured out for you is the new covenant in my blood (Lk 22:7-8,14-20).
Few doctrines have suffered a more systematic distortion than the Eucharist. This is ironic because the actual significance of the Eucharist is transparent and straightforward.
The Last Supper is based on the Passover. That's not to say it's identical to a Seder meal. It's an analogy, inviting comparison and contrast. As you'd expect, the Last Supper has a more specifically Christian significance. What do they share in common?
i) The Passover is a meal. A communal meal.
ii) The Passover illustrates a substitutionary principle, where the paschal lamb dies in place of the firstborn male.
iii) And you have the function of the blood. Not merely blood, but shed blood.
iv) Although it's a meal, the significance of the rite isn't based on consuming the elements. It's not about internalizing the meat and blood.
To the contrary, the lamb is exsanguinated, to avoid consumption of the blood. The blood is external to the celebrant.
The blood is not inside the celebrants. Rather, the celebrants are inside their homes, while the blood is painted on the doorway. The doorway is both an entrance and an exit. It represents the boundary between the home and the outside world.
The blood is a "sign" (v13). In the Passover rite, painting the doorway with blood forms an emblematic barrier, which prevents the Destroyer from transgressing the premises and executing the firstborn male within. This is a bit of divine theater. And object lesson.
So the blood has a protective function. It shields the firstborn male from divine judgment.
These basic principles carry over to the Last Supper:
i) The rite depicts the vicarious atonement of Christ. That's already clear from its background in the Passover, but is reinforced by allusions to the Suffering Servant (Isa 53:6,10,12, LXX).
The elements are somewhat different. There's an emphasis on bread and wine rather than the lamb. That's because Jesus will take the place of the lamb. No doubt the original Passover meal involved wine to wash down the solid food. But that ritual already had literal blood (the shed blood of the lamb), so there was no need for another element (wine) to symbolize blood–in contrast to the Last Supper.
ii) On analogy with the Passover, this is not about consuming blood. This is not about internalizing the body and blood of Christ.
Rather, the blood retains its protective significance. The wine represents the shed blood of Christ, anticipating the Crucifixion–just hours away. The bread represents the body of Christ, soon to suffer violent death. A sacrificial victim.
The blood of Christ shields the Christian from divine judgment. In this case, eschatological judgment (damnation) rather than physical death (the plague of the firstborn). The blood of Christ is "outside" us rather than inside us–like a protective barrier.