Solemnity of the Most Holy Trinity
The Blessed Trinity is the mystery of one God in three Divine Persons and it is the central mystery of our Christian faith and life. We call it a mystery because it cannot be understood by reason alone. God reveals himself as Father, Son and Holy Spirit. God has made it known to us.
The belief in one God comes to us through our Jewish ancestry. One of the primary prayers in the Jewish faith is taken from the Book of Deuteronomy:
“Hear, O Israel! The Lord is our God, the Lord alone! Therefore, you shall love the Lord, your God, with your whole heart, and with your whole being, and with your whole strength.” Deuteronomy 6: 4-5
God revealed himself to the Chosen People as the one and only God. Our Christian belief that there are three Divine Persons in one God came later, as it was revealed by the life and words of Jesus Christ. The Apostles came to understand that Christ was God.
“No one has ever seen God. The only Son, God, who is at the Father’s side, has revealed him.” John 1: 18
The Apostles later came to know and understand that the Holy Spirit was also God. In the Gospel of John, Jesus says, “When the Advocate comes whom I will send you from the Father, the Spirit of truth that proceeds from the Father, he will testify to me.“ John 16: 26
Before the Passion, Jesus promised to send the Holy Spirit as teacher, guide, and consoler. The appearance at Pentecost and at other events in the New Testament gives ample evidence of the Holy Spirit as the third Person of the Trinity.
The doctrine of the Trinity includes three truths of faith.
1.) The Trinity is One. We do not speak of three gods, but of one God. Each of the Persons is fully God. They are a unity of Persons in one divine nature.
2.) The Divine Persons are distinct from each other. Father, Son, and Holy Spirit are not three appearances or modes of God, but three identifiable persons, each fully God in a way distinct from the others.
3.) The Divine Persons are in relation to each other. The distinction of each is understood only in reference to the others. The Father can not be the Father without the Son, nor can the Son be the Son without the Father. The Holy Spirit is related to the Father and the Son who both send him forth.
All Christians are baptized in the name of the Father and of the Son and of the Holy Spirit. By the grace of Baptism, we are called to share in the life of the Blessed Trinity, here on earth in the obscurity of faith, and after death in eternal light. The Trinity enlightens all the other mysteries of faith.
SOURCE: United States Catholic Catechism for Adults