The Holy Spirit is the Third Person of the Most Blessed Trinity. With the Father and the Son, he is adored and glorified. The word “Spirit” comes from “the Hebrew word ruah, which, in its primary sense, means breath, air, wind” (Catechism of the Catholic Church [CCC] 691). While conversing with Nicodemus, Jesus likens the Holy Spirit to the wind (cf. Jn 3:8). To be sure, one can also attribute the terms “holy” and “spirit” to the Father and the Son, but by joining these words together, “Scripture, liturgy, and the theological language designate the inexpressible person of the Holy Spirit, without any possible equivocation with other uses of the terms ‘spirit’ and ‘holy’ ” (CCC 691).
Jesus also uses the term “Paraclete” for the Holy Spirit, which is a Greek word meaning “intercessor” or “comforter.” The Spirit is the one called to our side—to plead our cause for us. Therefore, he also is known as the Advocate. He is likewise called the Spirit of truth, the Spirit of the promise, the Spirit of adoption, the Spirit of Christ, the Spirit of the Lord, the Spirit of God, and the Spirit of Glory.
No one knows the thoughts of God except the Spirit of God (cf. 1 Cor 2:11). It is the Spirit who reveals God to us, that is, his Christ, who is his Word, his living Utterance. The Spirit does not speak on his own, but only what he hears from the Christ. Such divine self-effacing on the part of the Spirit “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” (CCC 687). The Spirit is also known through Sacred Scripture, Tradition, the Magisterium, the Church’s liturgies, individual prayer, the charisms and ministries of the Church, the apostolic and missionary life of the Church, and the witness of the saints.
For further reading on the Holy Spirit, please read CCC 687-688, 691-693. May God bless you.