In carefully planning and preparing the salvation of the whole human race the God of infinite love, by a special dispensation, chose for Himself a people to whom He would entrust His promises. First He entered into a covenant with Abraham (see Gen. 15:18) and, through Moses, with the people of Israel (see Ex. 24:8). To this people which He had acquired for Himself, He so manifested Himself through words and deeds as the one true and living God that Israel came to know by experience the ways of God with men. Then too, when God Himself spoke to them through the mouth of the prophets, Israel daily gained a deeper and clearer understanding of His ways and made them more widely known among the nations (see Ps. 21:29; 95:1-3; Is. 2:1-5; Jer. 3:17). The plan of salvation foretold by the sacred authors, recounted and explained by them, is found as the true word of God in the books of the Old Testament: these books, therefore, written under divine inspiration, remain permanently valuable. “For all that was written for our instruction, so that by steadfastness and the encouragement of the Scriptures we might have hope” (Rom. 15:4).
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 of the messianic kingdom, to announce this coming by prophecy (see Luke 24:44; John 5:39; 1 Peter 1:10), and to indicate its meaning through various types (see 1 Cor. 10:12). Now the books of the Old Testament, in accordance with the state of mankind before the time of salvation established by Christ, reveal to all men the knowledge of God and of man and the ways in which God, just and merciful, deals with men. These books, though they also contain some things which are incomplete and temporary, nevertheless show us true divine pedagogy. (1) These same books, then, 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.
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. (2) For, though Christ established the new covenant in His blood (see Luke 22:20; 1 Cor. 11:25), still the books of the Old Testament with all their parts, caught up into the proclamation of the Gospel, (3) acquire and show forth their full meaning in the New Testament (see Matt. 5:17; Luke 24:27; Rom. 16:25-26; 2 Cor. 14:16) and in turn shed light on it and explain it.
—from Dei Verbum (1965)