On the night Jesus was betrayed,
(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.” 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 22:19-20)
The same event is described in Matthew 26:26-28 and Mark 14:22-25, but only Luke uses the specific word “new” (καινὴ) to describe the covenant. Jesus instituted the first observance of the Lord’s Supper as a new covenant celebration, and Paul repeats this narrative in 1 Corinthians 11:25 about the cup of the new covenant in His blood as he instructs the church in Corinth about observing the Lord’s Supper.
There is more about the New Covenant in the New Testament as well. Paul also described his own work as a minister of a new covenant (2 Corinthians 3:6). The author of Hebrews says that Jesus is the mediator of this New Covenant (Hebrews 9:15; 12:14) and the coming of the New Covenant makes the first one, the Old one obsolete (Hebrews 8:13).
The New Covenant is unconditional, that is, it is a promise made by God, kept by God, and for God (see Jeremiah 31:31-4, and also Ezekiel 36:22-38). The fulfillment of the New Covenant is not based on what the Jews do or don’t do. In fact, the fulfillment of the New Covenant will be when God does in the Jews what they have definitely not done and could not do on their own.
To say, then, that the promises of the New Covenant are fulfilled in the church is to be sloppy with all the specific Old Testament promises. Parts of it are, but not the whole, not yet.
The better answer is that Jesus inaugurated and ratified the New Covenant. There is currently a Jewish remnant who are beneficiaries of the soul-saving elements of the New Covenant. There is currently an extension in salvation blessings of the New Covenant to the Gentiles. But the full/final fulfillment of the New Covenant has not yet happened. The Lord’s Table, especially this cup, remind us that all has been purchased, and to look forward to the accomplishment of all His Word.