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. [CCCC 45]

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). [CCCC 46]

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). [CCCC 47]

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. [CCCC 48]

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.

“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)

[CCCC 49]

We believe in one God in three persons (Trinity). “God is not solitude but perfect communion.” (Pope Benedict XVI, May 22, 2005).

Christians do not worship three different Gods, but one single Being that is threefold and yet remains one. We know that God is triune from Jesus Christ: He, the Son, speaks about his Father in heaven (“I and the Father are one”, Jn 10:30). He prays to him and sends us the Holy Spirit, who is the love of the Father and the son. That is why we are baptized “in the name of the Father and of the son and of the Holy Spirit” (Mt 28:19). [Youcat 35]

No. The fact that there are three persons (Trinity) in one God is a mystery. We know only through Jesus Christ that God is Trinitarian. 

Men cannot deduce the fact that God is a Trinity by means of their own reason. They acknowledge, however, that this mystery is reasonable when they accept God’s revelation in Jesus Christ. If God were alone and solitary, he could not love from all eternity. In the light of Jesus we find already in the Old Testament (for example, Gen 1:2; 18:2; 2 sam 23:2), indeed, even in all of creation, traces of God’s Trinitarian Being. [Youcat 36]

We revere God as Father first of all because he is the Creator and cares lovingly for his creatures. Jesus, the Son of God, has taught us, furthermore, to regard his Father as our Father and to address him as “our Father”.

Several pre-Christian religions had the divine title “Father”. Even before Jesus, the Israelites addressed God as their Father (Deut 32:6; Mal 2:10), realizing that he is also like a mother (Is 66:13). In human experience, father and mother stand for origin and authority, for what is protective and supportive. Jesus Christ shows us what God the Father is really like: “He who has seen me has seen the Father” (Jn 14:9). In the parable of the prodigal son, Jesus addresses the most profound human longings for a merciful father. [Youcat 37]

The Holy Spirit is the third person of the Holy Trinity and has the same divine majesty as the Father and the Son.

When we discover the reality of God in us, we are deal- ing with the working of the Holy Spirit. God sent “the spirit of his son into our hearts” (Gal 4:6), so that he might fill us completely. In the Holy Spirit a Christian finds profound joy, inner peace, and freedom. “For you did not receive the spirit of slavery to fall back into fear, but you have received the spirit of sonship [in whom] we cry, ‘Abba! Father!’” (Rom 8:15b). In the Holy Spirit, whom we receive in Baptism and Confir- mation we are permitted to call God “Father”. [Youcat 38]

Jesus of Nazareth is the Son, the second divine person mentioned when we pray, “In the name of the Father and of the Son and of the Holy Spirit”  (Mt 28:19).   

Jesus was either an imposter who made himself lord of the sabbath and allowed himself to be addressed with the divine title “lord”—or else he was really God. The scandal came when he forgave sins. In the eyes of his contemporaries, that was a crime deserving death. Through signs and miracles, but especially through the resurrection, his disciples recognized who Jesus is and worshipped him as Lord. That is the faith of the  Church. [Youcat 39]