Catechism in a Year
Sunday, April 22, 2018
136. What does the Church mean when she confesses: "I believe in the Holy Spirit"?
To believe in the Holy Spirit is to profess faith in the Third Person of the Most Holy Trinity who proceeds from the Father and the Son and "is worshipped and glorified with the Father and the Son". The Spirit is "sent into our hearts" (Galatians 4:6) so that we might receive new life as sons of God.
Further reading: CCC 683-686
137. Why are the missions of the Son and the Holy Spirit inseparable?
In the indivisible Trinity, the Son and the Spirit are distinct but inseparable. From the very beginning until the end of time, when the Father sends his Son he also sends his Spirit who unites us to the Christ in faith so that as adopted sons we can call God "Father" (Romans 8:15). The Spirit is invisible but we know him through his actions, when he reveals the Word to us and when he acts in the Church.
Further reading: CCC 687-690, 742-743
These excerpts are from the Compendium of the CCC.