The Old Testament is very important for Christians. The Catechism of the Catholic Church explains:
The Old Testament is an indispensable part of Sacred Scripture. Its books are divinely inspired and retain a permanent value, for the Old Covenant has never been revoked.
Indeed, "the economy of the Old Testament was deliberately so oriented that it should prepare for and declare in prophecy the coming of Christ, redeemer of all men." "Even though they contain matters imperfect and provisional," the books of the Old Testament bear witness to the whole divine pedagogy of God’s saving love: these writings "are a storehouse of sublime teaching on God and of sound wisdom on human life, as well as a wonderful treasury of prayers; in them, too, the mystery of our salvation is present in a hidden way."
Christians venerate the Old Testament as true Word of God. The Church has always vigorously opposed the idea of rejecting the Old Testament under the pretext that the New has rendered it void (Marcionism). (CCC 121-123)