The Catechism in Questions & Answers

207. Was the Holy Spirit also active in the time of the Old Testament?

Yes. Holy Scripture mentions the activity of the Holy Spirit on many occasions. The Holy Spirit inspired human beings so that they could act as instruments in accordance with the will of God. For example, He was active in the prophets of the Old Testament and spoke through them. Through the Holy Spirit, promises concerning the coming of the Messiah were brought forth.