The mystery of the Most Holy Trinity is the central mystery of the Christian faith and life. It is the mystery of God in himself. It is therefore the source of all the other mysteries of faith, the light that enlightens them….The whole history of salvation is identical with the history of the way and the means by which the one true God, Father, Son, and Holy Spirit, reveals himself to men (CCC 234).
By calling God “Father”, the language of faith indicates two main things: that God is the first origin of everything and transcendent authority; and that he is at the same time goodness and loving care for all his children (CCC 239).
Jesus revealed that God is Father in an unheard of sense: he is Father not only in being Creator; he is eternally Father by his relationship to his only Son who, reciprocally, is Son only in relation to his Father…. For this reason the apostles confess Jesus to be the Word … “and the Word was God” (Jn1:1), as “the image of the invisible God” (Col 1:15); as the “radiance of the glory of God and the very stamp of his nature” (Heb 1:3) (CCC 240, 241).
Before his Passover, Jesus announced the sending of “another Paraclete” (Advocate), the Holy Spirit (Jn 14:17). At work since creation, having previously “spoken through the prophets” (Nicene Creed), the Spirit will now be with and in the disciples to teach them and guide them “into all the truth” (Jn 16:13) (CCC 243).