16 Believe thou also in the Holy Ghost, and hold the same opinion concerning Him, which you have received to hold concerning the Father and the Son, and follow not those who teach blasphemous things of Him. But learn thou that this Holy Spirit is One, indivisible, of manifold power; having many operations, yet not Himself divided; Who knows the mysteries, Who searches all things, even the deep things of God: Who descended upon the Lord Jesus Christ in form of a dove; Who wrought in the Law and in the Prophets; Who now also at the season of Baptism seals your soul; of Whose holiness also every intellectual nature has need: against Whom if any dare to blaspheme, he has no forgiveness, neither in this world, nor in that which is to come: “Who with the Father and the Son together” is honoured with the glory of the Godhead: of Whom also thrones, and dominions, principalities, and powers have need. For there is One God, the Father of Christ; and One Lord Jesus Christ, the Only-begotten Son of the Only God; and One Holy Ghost, the sanctifier and deifier of all, Who spoke in the Law and in the Prophets, in the Old and in the New Testament.
Source: Catechetical Lectures (New Advent)