105. Who is the Holy Ghost (Holy Spirit)?
The Holy Ghost is God and the third Person of the Blessed Trinity.
(a) The Holy Ghost is also called the Holy Spirit, the Paraclete, the Advocate, the Spirit of Truth, the Spirit of God, and the Spirit of Love.
106. From whom does the Holy Ghost proceed?
The Holy Ghost proceeds from the Father and the Son.
(a) The Holy Ghost does not proceed from the Father and the Son by spiritual generation. Only the Son proceeds from the Father by generation. This is one of the mysterious truths that we know only from revelation.
107. Is the Holy Ghost equal to the Father and the Son?
The Holy Ghost is equal to the Father and the Son, because He is God.
(a) Because of the oneness of nature in the Blessed Trinity, the Father is entirely in the Son and in the Holy Ghost; the Son is entirely in the Father and in the Holy Ghost; and the Holy Ghost is entirely in the Father and in the Son. No one of the three divine Persons is outside the other, for none precedes the other in eternity, nor surpasses the other in power, nor exceeds the other in any way. This indwelling of one divine Person in the others is called circumincession.
108. What does the Holy Ghost do for the salvation of mankind?
The Holy Ghost dwells in the Church as the source of its life and sanctifies souls through the gift of grace.
(a) Although the sanctification of mankind, like all other outward works of God, is performed by all three Persons of the Blessed Trinity, it is attributed to the Holy Ghost, the third Person. The sanctification of mankind is attributed to the Holy Ghost because He is the love of the Father and the Son and because the sanctification of man by grace shows forth God's boundless love.
The Baltimore Catechism, Lesson 9.