“And likewise the cup after they had eaten, saying, "This cup that is poured out for you is the new covenant in my blood." - Luke 22:20
When Jesus says, this is the cup of the covenant, what does that mean? What is a covenant? A covenant is an agreement between two parties. So Jesus is saying this is the cup of the new agreement. So if this is the new agreement, what is the old agreement? The old agreement is the agreement that God had with Israel when He brought them out of Egypt. This leads us to another question, what was wrong with the old agreement.
“Behold the days are coming, declares the Lord, when I will make a new covenant with the house of Israel and the house of Judah, not like the covenant that I made with their fathers on the day when I took them by the hand to bring them out of the land of Egypt, my covenant that they broke, though I was their husband, declares the Lord.”- Jeremiah 31:31-32
The problem with the old covenant was that Israel committed adultery and broke the agreement. The covenant was breached, and therefore it was no longer any good. So how does God respond? What is the old adage, if you want something done right, you do it yourself, and this is exactly what God says he will do.
For this is the covenant that I will make with the house of Israel after those days, declares the Lord: I will put my law within them, and I will write it on their hearts. And I will be their God, and they shall be my people.” - Jeremiah 31:32-33
This new agreement will have God holding up both sides. He will, of course, hold up His end, but he will also hold up the people's end. He will do this by putting his law within them and write it on their hearts. What does that mean? It means that the relationship between God and the people is not regulated by outward influence, but instead by an inward desire. God will change the people's affections towards Him. No longer will you obey God because you have to, but because you want to. He is no longer your Master, but he is your Father. This is the new agreement, this is the new covenant.
And this is what is in the cup, the cup that is filled with the blood of Jesus. When Jesus says drink of the cup of the new covenant, He is saying drink my blood which achieves the new agreement. Drink of the cup that fulfills the promise of Jeremiah 31. Drink the reality that I am God and my death achieves your side of the the bargain. Jesus is saying my death puts the law within you and writes it on your heart.
Few people recognize this depth of the cup of the blood of Jesus. For it achieves far more than most people recognize. Jesus' death is not just providing you an opportunity to be saved. It is the actual blood ransom for your love for God. His death purchased your faith. This is why in Hebrews 12:2 it says that Jesus is the founder of our faith. Our faith does not originate from us, but from the death of Christ. This is why we will sing in heaven:
“Worthy are you to take the scroll and to open its seals, for you were slain, and by your blood you ransomed people for God from every tribe and language and people and nation, and you have made them a kingdom and priests to our God, and they shall reign on the earth.” - Revelation 5:9-10
This should not be a song only for heaven. It should be a song for today. We should sing it as often as we can, but especially when partaking in the worship that is the Lord's Supper.