Catechism of the Catholic Church §685 — excerpt
To believe in the Holy Spirit is to profess that the Holy Spirit is one of the persons of the Holy Trinity, consubstantial with the Father and the Son: "with the Father and the Son he is worshipped and glorified."
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: Nicene Creed · see above, par. 465