Catechism of the Catholic Church §687 — excerpt
"No one comprehends the thoughts of God except the Spirit of God." Now God's Spirit, who reveals God, makes known to us Christ, his Word, his living Utterance, but the Spirit does not speak of himself.
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.