Catechism of the Catholic Church §152 — excerpt
One cannot believe in Jesus Christ without sharing in his Spirit. It is the Holy Spirit who reveals to men who Jesus is. For "no one can say "Jesus is Lord", except by the Holy Spirit", who "searches everything, even the depths of God.
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.