Okay, so it’s increasingly clear that I won’t have time anytime soon either to research or to write a thorough, comprehensive post about the role of the Holy Spirit in the Catholic tradition. But for several reasons, I thought it important that I go ahead and move on this, if only in spurts and gasps. A dear friend who is questioning his faith recently posed some questions about the Holy Spirit that seemed timely to this post. Today at a used bookstore I picked up a second, paperback copy of the Catechism, for me to carry around and to write in. And tonight at RCIA, we had Catholic trivia night, and my team won. (We beat the team behind us by only five points. The answer that pushed us over the top was to the bonus question: What was Pope John XXIII’s family name? Roncalli.) We each won copies of the Compendium of the Catechism. I promptly dropped mine in a puddle, but I dried it quickly, and I don’t think it’s too damaged. Anyway — the Catechism is in my hand and on my mind, so now seemed a good time to take a crack at this.
Again, this won’t be comprehensive, complete, or well-studied. These writings represent my ongoing process to wrap my head around these concepts. But through working through this and refining it, from both the Catholic and Protestant positions, I hope to come up with something worth presenting.
The primary role of the Holy Spirit in the Catholic understanding is to reveal Christ to the believer. The Catechism:
[The] knowledge of faith is possible only in the Holy Spirit: to be in touch with Christ, we must first have been touched by the Holy Spirit. He comes to meet us and kindles faith in us. By virtue of our Baptism, the first sacrament of the faith, the Holy Spirit in the Church communicates to us, intimately and personally, the life that originates in the Father and is offered to us in the Son. 
What about, though, those nonbelievers who haven’t yet received baptism? How do they receive faith? Presumably the Holy Spirit also gives them grace and faith to believe, to be converted and baptized. St. Paul says,
if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved. . . . How then will they call on him in whom they have not believed? And how are they to believe in him of whom they have never heard? And how are they to hear without someone preaching? . . . So faith comes from hearing, and hearing through the word of Christ. (Romans 10:9-17, ESV)
And also in Ephesians, in a favorite verse of Protestants:
For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast. (Ephesians 2:8-9, ESV)
So God gives us the gift of faith, through the Holy Spirit, and it is by this faith that we believe. The Catechism continues:
Through his grace, the Holy Spirit is the first to awaken faith in us and to communicate to us the new life, which is to “know the Father and the one whom he has sent, Jesus Christ” (John 17:3). 
“No one comprehends the thoughts of God except the Spirit of God” (1 Corinthians 2:11). 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” (John 16:13). 
So we hear the Word, as revealed to us by the Holy Spirit. He disposes us to welcome Christ in faith. The Spirit grants us the gift of faith to believe. This seems to be the most essential work of the Holy Spirit, regardless of what Christian tradition you are coming from.