Sacrament & Document: What is the New Testament?
When we use the term “New Testament,” we usually just mean the last twenty-seven books of the Holy Bible, the ones written by the Apostles and their companions in the first century. But that’s not what the term meant from the beginning. Long before those books were even written, Jesus used that phrase to refer to the Eucharist. It was the "New Covenant," the "New Testament," in his blood. Christians later extended the phrase to cover the books; but they did so precisely because these were the books that could be read at Mass.
Dr. Scott Hahn