Catechism of the Catholic Church to get a good summary of what Hahn focuses in on in chapter 7. It looks at how the the New Testament authors utilized the Old Testament writings. This is essential because it shows the connection between the old and the new, promise and fulfillment.
"By virtue of their office in the nascent Church, the Apostles definitively interpreted the word of the Old Testament in light of the dispensation of the New Testament (62)."
"In the Old Testament Scriptures the entire New Testament was foreshadowed. In the New Testament dispensation, all the Old Testament Scriptures were fulfilled. As Saint Augustine put it: 'The New Testament is concealed in the Old, and the Old Testament is revealed in the New (62).'"