From the Compendium of the Catechism of the Catholic Church:
44. What is the central mystery of Christian faith and life?
The central mystery of Christian faith and life is the mystery of the Most Blessed Trinity. Christians are baptized in the name of the Father and of the Son and of the Holy Spirit.
45. Can the mystery of the Most Holy Trinity be known by the light of human
God has left some traces of his trinitarian being in creation and in the Old Testament but his inmost being as the Holy Trinity is a mystery which is inaccessible to reason alone or even to Israel’s faith before the Incarnation of the Son of God and the sending of the Holy Spirit. This mystery was revealed by Jesus Christ and it is the source of all the other mysteries.
46. What did Jesus Christ reveal to us about the mystery of the Father?
Jesus Christ revealed to us that God is “Father,” not only insofar as he created the universe and the mankind, but above all because he eternally generated in his bosom the Son who is his Word, “ the radiance of the glory of God and the very stamp of his nature” ( Hebrews 1:3).
“O my God, Trinity whom I adore...grant my soul peace; make it your heaven, your beloved dwelling, and the place of your rest. May I never abandon you there, but may I be there, whole and entire, completely vigilant in my faith, entirely adoring, and wholly given over to your creative action.” (Blessed Elizabeth of the Trinity)
47. Who is the Holy Spirit revealed to us by Jesus Christ?
The Holy Spirit is the third Person of the Most Blessed Trinity. He is God, one and equal with the Father and the Son. He “proceeds from the Father” ( John 15:26) who is the principle without a principle and the origin of all trinitarian life. He proceeds also from the Son (Filioque) by the eternal Gift which the Father makes of him to the Son. Sent by the Father and the Incarnate Son, the Holy Spirit guides the Church “to know all truth” ( John 16:13).
48. How does the Church express her trinitarian faith?
The Church expresses her trinitarian faith by professing a belief in the oneness of God in whom there are three Persons: Father, Son, and Holy Spirit. The three divine Persons are only one God because each of them equally possesses the fullness of the one and indivisible divine nature. They are really distinct from each other by reason of the relations which place them in correspondence to each other. The Father generates the Son; the Son is generated by the Father; the Holy Spirit proceeds from the Father and the Son.
49. How do the three divine Persons work?
Inseparable in their one substance, the three divine Persons are also inseparable in their activity. The Trinity has one operation, sole and the same. In this one divine action, however, each Person is present according to the mode which is proper to him in the Trinity.