Dei Verbum

In nn. 15 and 16 it deals with, “The principal purpose to which the plan of the old covenant was directed was to prepare for the coming of Christ, the redeemer of all, and the messianic kingdom, to announce this coming by prophecy and to indicate its meaning by various types.” The books of the Old Testament show true divine teaching even though they contain incomplete and temporary things as well. They “give expression to a lively sense of God”, “contain a store of sublime teachings about God, sound wisdom about human life, and a wonderful treasury of prayers, and in them the mystery of our salvation is present in a hidden way. Christians should receive them with reverence.”

Because of that understanding of the Old Testament we can see the importance of both Testaments to our faith. N. 16 states it well:

“God, the inspirer and author of both Testaments, wisely arranged that the New Testament be hidden in the Old and the Old be made manifest in the New. For, though Christ established the new covenant in His blood, still the books of the Old Testament with all their parts, caught up into the proclamation of the Gospel, acquire and show forth their full meaning in the New Testament and in turn shed light on it and explain it.”