In Luke 22:20 during the Last Supper, why is the cup of wine (Jesus blood) referred to as the New Covenant? What was the Old Covenant?
We can appreciate the difference between the Old- and New Covenants by paying attention to one word in the incredibly significant words Jesus spoke to his disciples on the eve of the Passover in the upper room, just prior to his arrest, trial, and crucifixion.
That word is my.
"This cup which is poured out for you is the new covenant in My blood" (Luke 22:20 NASB Updated; emphasis is mine; italics and bolding are mine).
Put simply, the New Covenant was ratified with Jesus' blood, whereas the Old Covenant was ratified with the blood of animals. From Hebrews:
when Christ appeared as a high priest of the good things to come, He entered through the greater and more perfect tabernacle, not made with hands, that is to say, not of this creation; and not through the blood of goats and calves, but through His own blood, He entered the holy place once for all, having obtained eternal redemption. For if the blood of goats and bulls and the ashes of a heifer sprinkling those who have been defiled sanctify for the cleansing of the flesh, how much more will the blood of Christ, who through the eternal Spirit offered Himself without blemish to God, cleanse your conscience from dead works to serve the living God? For this reason He is the mediator of a new covenant, so that, since a death has taken place for the redemption of the transgressions that were committed under the first covenant, those who have been called may receive the promise of the eternal inheritance (9:11-15 NASB; italics and bolding are mine).
From Michael Hunt's article, "The Ratification of the Sinai Covenant," we read
"On the night Christ first offered the faithful His Body and Blood He spoke of the New Covenant in His blood. As people who were bound to Yahweh through the blood of the sacrifice poured out on Yahweh's altar and sprinkled on the people in the covenant ratification ceremony at Mt. Sinai all those centuries earlier, the words Jesus spoke were a signal to His disciples that what was begun in that Old Covenant was now completed in a New and everlasting Covenant that had been promised by the prophet Jeremiah. Lord, may we have the courage to keep our oaths of allegiance as we commit ourselves, like the faithful of the community of the Sinai Covenant, saying: "We shall do everything that Christ has said "we shall obey" the covenant that is our inheritance, not through the blood of animals but through the blood of our precious Savior. Send Your Holy Spirit to guide us, Lord, in our study of the first corporate covenant that was a precursor to our universal New Covenant in Christ Jesus. We pray in the name of God the Father, the Son, and the Holy Spirit. . . . That is why even the earlier covenant was inaugurated with blood, and why, after Moses had promulgated all the commandments of the Law to the people he took the calves' blood, the goats' blood and some water, and with these he sprinkled the book itself and all the people, using scarlet wool and hyssop: saying as he did so: 'This is the blood of the covenant that God has made with you.'"
So the ratification of the Old Covenant involved blood, but it also involved the obedience of the Hebrews to "all the words of the LORD and all the ordinances" God gave Moses on the mountain at Sinai. God actually wrote with his own finger "the law and the commandment" on tablets of stone which Moses then conveyed to the people (see Exodus 24, particularly vv.3 and 12).
If I were to summarize the Old Covenant in two words, they would be blood and obedience, which begs the question "Which blood and whose obedience?" The answer: the blood was the blood of animal sacrifices and the obedience was that of the Hebrews.
As for the New Covenant, when we ask the same question, "Which blood and whose obedience?" the answer is Jesus' blood and Jesus' obedience.
In conclusion, whereas the Old Covenant required repeated sacrifices because the Hebrews' obedience was only partial and was manifested in fits and starts, the New Covenant required only the perfect obedience of the perfectly sinless Lamb of God. Through Jesus' complete and utter obedience to the will of his Father, Jesus once and for all took away our sin by the sacrifice of himself (Hebrews 9:26b NIV).
Perhaps the most important thing to know about the new covenant is that, like the Sinai covenant (which is the old/first covenant in relation to the new covenant) it is made only with the houses of Israel and Judah:
Hebrews 8: 6Now, however, Jesus has received a far superior ministry, just as the covenant He mediates is superior and is founded on better promises. 7For if that first covenant [the Sinai covenant] had been without fault, no place would have been sought for a second. 8But when God found fault with the people, He said:
“Behold, the days are coming, says the Lord, when I will make a new covenant with the house of Israel and with the house of Judah. 9It will not be like the covenant I made with their fathers when I took them by the hand to lead them out of the land of Egypt, because they did not abide by My covenant, and I disregarded them, says the Lord. 10This is the covenant I will make with the house of Israel after those days, says the Lord. I will put My laws in their minds, and inscribe them on their hearts. And I will be their God, and they will be My people. 11No longer will each one teach his neighbor or his brother, saying, ‘Know the Lord,’ because they will all know Me, from the least of them to the greatest. 12For I will forgive their iniquities, and remember their sins no more.”b 13In speaking of a new covenant, He has made the first one obsolete; and what is obsolete and aging will soon disappear.
The purpose, or at least an important part was to deal with the transgressions committed under the first covenant:
Heb 9:15Therefore Christ is the mediator of a new covenant, so that those who are called may receive the promised eternal inheritance, now that He has died to redeem [release] them from the transgressions committed under the first covenant.
This is why this covenant is explained "To the Hebrews" and why the covenant is celebrated as part of the Passover which is also specific to the Jews. Jesus did NOT institute "Communion" for gentiles.
The cup of wine symbolizes the blood that ratified the covenant just as blood ratified the first:
NASB Exodus 24: 6Moses took half of the blood and put it in basins, and the other half of the blood he sprinkled on the altar. 7Then he took the book of the covenant and read it in the hearing of the people; and they said, "All that the LORD has spoken we will do, and we will be obedient!" 8So Moses took the blood and sprinkled it on the people, and said, "Behold the blood of the covenant, which the LORD has made with you in accordance with all these words."
NIV Heb 9: 15For this reason Christ is the mediator of a new covenant, that those who are called may receive the promised eternal inheritance—now that he has died as a ransom to set them free from the sins committed under the first covenant.
16In the case of a will,d it is necessary to prove the death of the one who made it, 17because a will is in force only when somebody has died; it never takes effect while the one who made it is living. 18This is why even the first covenant was not put into effect without blood. 19When Moses had proclaimed every command of the law to all the people, he took the blood of calves, together with water, scarlet wool and branches of hyssop, and sprinkled the scroll and all the people. 20He said, “This is the blood of the covenant, which God has commanded you to keep.”