3 There is One Only Holy Ghost, the Comforter; and as there is One God the Father, and no second Father;— and as there is One Only-begotten Son and Word of God, who has no brother—so is there One Only Holy Ghost, and no second spirit equal in-honour to Him. Now the Holy Ghost is a Power most mighty, a Being divine and unsearchable; for He is living and intelligent, a sanctifying principle of all things made by God through Christ. He it is who illuminates the souls of the just; He was in the Prophets, He was also in the Apostles in the New Testament. Abhorred be they who dare to separate the operation of the Holy Ghost! There is One God, the Father, Lord of the Old and of the New Testament: and One Lord, Jesus Christ, who was prophesied of in the Old Testament, and came in the New; and One Holy Ghost, who through the Prophets preached of Christ, and when Christ had come, descended, and manifested Him.
Source: Catechetical Lectures (New Advent)