Dominum et Vivificantem §17

¹⁷ Here it must be emphasized that clearly the "spirit of the Lord" who rests upon the future Messiah is above all a gift of God for the person of that Servant of the Lord. But the latter is not an isolated and independent person, because he acts in accordance with the will of the Lord, by virtue of the Lord's decision or choice. Even though in the light of the texts of Isaiah the salvific work of the Messiah, the Servant of the Lord, includes the action of the Spirit which is carried out through himself, nevertheless in the Old Testament context there is no suggestion of a distinction of subjects, or of the Divine Persons as they subsist in the mystery of the Trinity, and as they are later revealed in the New Testament. Both in Isaiah and in the whole of the Old Testament the personality of the Holy Spirit is completely hidden: in the revelation of the one God, as also in the foretelling of the future Messiah.