Jeremiah, writing amid the wreck of the old order, promised something without precedent: 'Behold the days shall come, saith the Lord, and I will make a NEW COVENANT with the house of Israel, and with the house of Juda: not according to the covenant which I made with their fathers... I will give my law in their bowels, and I will write it in their heart... for I will forgive their iniquity, and I will remember their sin no more' (Jeremiah 31:31–34). It is the only occurrence of the phrase 'new covenant' in the Old Testament.
At the Last Supper, in the upper room on this hill, Christ took the chalice and deliberately reached for Jeremiah's phrase and Moses' words at Sinai ('the blood of the covenant,' Exodus 24:8) at once: 'This is the chalice, the NEW TESTAMENT IN MY BLOOD, which shall be shed for you' (Luke 22:20; 1 Corinthians 11:25). One sentence, two prophecies, sealed in His own blood the following afternoon.
The Epistle to the Hebrews quotes Jeremiah 31 at full length — the longest Old Testament citation in the New — and concludes: 'In saying a new, he hath made the former old' (Hebrews 8:8–13). The Church hears the fulfillment renewed at every Mass, in the words of consecration: novi et aeterni testamenti — the new and eternal covenant.