The New Covenant was predicted by Old Covenant prophets. It had a beginning point, it has very specific terms, and it is a major subject of the New Testament scriptures. It is at the heart of the earliest conflict to beset the churches of Jesus Christ. These conflicts still continue, today.
Though the terms "New Testament" and "New Covenant" mean the same thing, we use them differently. The New Testament is composed of 27 books that start with Christ's birth in Bethlehem, but the New Covenant was not in effect at His birth. Nor was it in effect when Christ gave the sermon on the mount or at His triumphal entry into Jerusalem when they yelled, "Hosannah" to the messiah. The New Covenant did not come into effect until after the death of Christ.
On this question, God tells us that, "A covenant is valid only when men are dead; it is never in force while the one who made it is alive." The New Covenant came into effect after Christ's death. It could not come into effect until Christ died (Hebrews 9:15-17). So on the night before Christ died, He spoke words which echoed the words of Moses at the beginning of the Old Covenant. The Lord Jesus said, "This is my blood of the covenant, which is poured out for many for forgiveness of sins" (Matthew 26:27-28).