What significance does the Old Testament have for Christians?
In the Old Testament God reveals himself as the Creator and preserver of the world and as the leader and instructor of mankind. The Old Testament books are also God’s Word and Sacred Scripture. Without the Old Testament, we cannot understand Jesus.
In the Old Testament a great history of learning the faith begins, which takes a decisive turn in the New Testament and arrives at its destination with the end of the world and Christ’s second coming. The Old Testament is far more than a mere prelude for the New. The commandments and prophecies for the people of the Old Covenant and the promises that are contained in it for all men were never revoked. In the books of the Old Covenant we find an irreplaceable treasure of prayers and wisdom; in particular, the Psalms are part of the Church’s daily prayer.
CCC section (121-123)