Catechism of the Catholic Church §140 — excerpt
The unity of the two Testaments proceeds from the unity of God's plan and his Revelation. the Old Testament prepares for the New and the New Testament fulfils the Old; the two shed light on each other; both are true Word of God.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.