From the beginning until "the fullness of time," The joint mission of the Father's Word and Spirit remains hidden, but it is at work. God's Spirit prepares for the time of the Messiah. Neither is fully revealed but both are already promised, to be watched for and welcomed at their manifestation. So, for this reason, when the Church reads the Old Testament, she searches there for what the Spirit, "who has spoken through the prophets," wants to tell us about Christ.
By "prophets" the faith of the Church here understands all whom the Holy Spirit inspired in the composition of the sacred books, both of the Old and the New Testaments. Jewish tradition distinguishes first the Law (the five first books or Pentateuch), then the Prophets (our historical and prophetic books) and finally the Writings (especially the wisdom literature, in particular the Psalms).
Source: Catechism of the Catholic Church (Vatican.va)