Chapter 10 — Part I: The Creed — Article VIII: I Believe in the Holy Ghost
Section 13. Again, the Sacred Scriptures join the Person of the Holy Ghost to those of the Father and the Son, as, for example, when Baptism is commanded to be administered in the name of the Father, and of the Son, and of the Holy Ghost. There is thus no room left us of doubting the truth of this mystery. For if the Father is God, and the Son God, we must admit that the Holy Ghost, who is united with Them in the same degree of honour, is also God.