is the central mystery of the Christian faith and life.
The Trinity is the mystery of God in himself. It is therefore the source of all the other mysteries of faith, the light that enlightens them. It is the most fundamental and essential teaching in the hierarchy of the truths of faith. The whole history of salvation is identical with the history of the way by which the one true God, Father, Son and Holy Spirit, reveals himself to people and reconciles and unites with himself those who turn away from sin
The Revelation of God as Trinity
The trinity is a mystery of faith, one of the mysteries that are hidden in God, which could never be known unless they are revealed by God. God has left traces of his Trinitarian being in his revelation throughout the Old Testament. But his inmost being as holy Trinity is a mystery that is inaccessible to reason alone or even to Israel's faith before the Incarnation of God's Son and the sending of the Holy Spirit.
God as Father in the Old Testament
In Israel, God is called Father inasmuch as He is the Creator of the world. Even more, God is Father because of the covenant and the gift of the law to Israel. He is also called Father of the poor, the orphaned, and the widowed, who are under His loving protection.
The Father revealed by the Son
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.
The Apostles confess Jesus is the Word:
“In the beginning was the Word, and the Word was with God, and the Word was God,” He is “the image of the invisible God,” the “radiance of the Glory of God, and the very stamp of His nature (John 1:1, Col.1:15; Hebrews 1:3)
Councils’ Teaching of the Trinity
Following the Apostolic tradition, the Church confessed at the First Ecumenical Council at Nicaea, in 325, that Jesus the Son is one only God with the Father and the Holy Spirit. The Second Ecumenical Council, held at Constantinople in 381, kept this expression in its formulation of the Nicene Creed and confessed the only-begotten Son of God, eternally begotten of the Father, Light from light, true God from true God, begotten not made, consubstantial with the Father. (DB691)
The Sending of the Holy Spirit
Before His Passover, Jesus announced the sending of another Paraclete (Advocate), the Holy Spirit. At work since creation, having previously spoken through the prophets, the Spirit will now be with and in the disciples, to teach them and guide them into all truth. The Holy Spirit is thus revealed as another divine Person with Jesus and the Father.
The eternal origin of the Holy Spirit is revealed in His mission in time. The Spirit is sent to the Apostles and to the Church both by the Father and the Son. The sending of the Person of the Spirit after Jesus’ glorification reveals in its fullness the mystery of the Holy Trinity.
