The God of Abraham, Isaac and Jacob is the one, only, living and true God, and He gradually revealed himself to Abraham and his descendants in order to make of them a chosen people. Through Moses, God gave to the children of Israel the high privilege and sacred duty of announcing to all the nations that there is but one God — “Hear, O Israel: The Lord our God, the Lord is one.” (Deuteronomy 6.4) The gradual revelation of the truth about the living God reached its fulfillment in the Lord Jesus, true God and true man, and it is Christ who completed the self-disclosure of the one God by revealing Him in his unity to be a communion of three divine Persons: Father, Son, and Holy Spirit. The Catechism of the Catholic Church describes the mystery of the Holy Trinity in this way:
“Christians are baptized in the Name of the Father and of the Son and of the Holy Spirit; not in their names, for there is only one God, the almighty Father, His only Son, and the Holy Spirit: the Most Holy Trinity.
“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 other mysteries of faith, the light that enlightens them. It is the most fundamental and essential teaching in the hierarchy of truths of faith. 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 and reconciles and unites with Himself those who turn away from him.
“The Trinity is a mystery of faith in the strict sense, one of the mysteries that are hidden in God, which can never be known unless they are revealed by God. To be sure, God has left traces of His Trinitarian being in His work of creation and 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.” (Catechism of the Catholic Church, numbers 233, 234, and 237)
This Solemnity of the Most Holy Trinity always falls on the first Sunday after Pentecost, drawing the gaze of our contemplation to the mystery of God in Himself at the conclusion of Eastertide.
Glory be to the Father, and to the Son, and to the Holy Spirit; as it was in the beginning, is now, and ever shall be, world without end. Amen.