The Holy Spirit
From the Catechism of the Catholic Church, Simplified
Saying "Jesus is Lord" (683)
Only through the Holy Spirit can a person be in touch with Christ (1 Cor 12:3). By Baptism, the Spirit gives us the life of the Father which is offered in the Son. "Those who bear God's Spirit are led to the Son, and the Son presents them to the Father who confers incorruptibility on them" (St. Irenaeus).
The Spirit - The Last Person Revealed (684)
Although the Spirit is the first to communicate new life, the Spirit was the last of the persons to be revealed. St. Gregory of Nazianzus explained this pedagogy of divine "condescension" when he wrote that the Father was revealed in the Old Testament and the Son in the New. It was not prudent to reveal the Son's divinity until the Father's divinity was confessed. So, also, the Spirit's divinity was withheld until the Son's divinity was believed.
The Spirit Works in Our Midst (685-686)
The Nicene Creed teaches that the Holy Spirit is a person of the Trinity, of one substance with the Father and the Son. This section will study how the Spirit does his works (the divine "economy").
In this "final age" (ushered in by Christ's redeeming Incarnation), the divine plan is accomplished by the outpouring of the Spirit. He brings about the Church, the Communion of Saints, the forgiveness of sins, the resurrection of the body and life everlasting.