"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. The Spirit who "has spoken through the prophets" makes us hear the Father's Word, but we do not hear the Spirit himself. We know him only in the movement by which he reveals the Word to us and disposes us to welcome him in faith. The Spirit of truth who "unveils" Christ to us "will not speak on his own." Such properly divine self-effacement explains why "the world cannot receive (him), because it neither sees him nor knows him," while those who believe in Christ know the Spirit because he dwells with them.
Source: Catechism of the Catholic Church (Vatican.va)