Among the perfections of God is that He is one and there is no other. Furthermore, within Himself God is perfect love, meaning that He is relational within Himself. It is by way of His “personal” nature that such relationship of love within Him exists – and it is accordingly that we can enter into an overview of God as a Trinity of persons.
The Holy and Blessed Trinity is the central mystery of our Catholic Faith – as it pertains directly to God Himself as He is within Himself. Reminding ourselves that God’s interior life is made know to us by His self-revelation handed down through the Scripture and the Sacred Tradition, the Trinity is seen to be revealed throughout the Scriptures, showing us God as the Father, that He is the Son, and that He is the Holy Spirit. Among the Scripture passages, in Matthew 28:20, when Jesus commissions the apostles to go forth, He reveals the Trinity in very direct fashion, telling the apostles to baptize “In the name of the Father and of the Son and of the Holy Spirit.” Each of these three persons is fully and equally God Himself; and each one is distinct from one another. Though no Scripture passage uses the word “trinity,” the reality of the life of God as a trinity of persons is revealed in multiple places, particularly in various places where Jesus speaks of His own relationship with the Father and the Holy Spirit. This truth of God as a trinity of persons requires our “supernatural” faith – as it is a mystery beyond our full ability to comprehend, even if we are capable of understanding something of its fullness of truth.
On the most basic level to show the unity in the Trinity, note that Jesus’ words in the above quote from Matthew 28:20 say to baptize in the “name” of; and not “names” of. In keeping with our goal of simplicity in this series, The New St. Joseph Baltimore Catechism, No. 1, in Lesson 3, offers us the following explanation (at an elementary level): “We may compare the three Persons in one God to three persons in a human family: father, mother, and child. But the big difference is that three Persons in God are ONE GOD. God the Father is the first Person. God the Son, Jesus Himself, is the second Person. And god the Holy Spirit is the third Person. THREE PERSONS, BUT ONLY ONE GOD.”
Within the interior life of God, we are shown that the Father loves the Son and the Son loves the Father – and the Holy Spirit proceeds forth from their love (Catechism of the Catholic Church, paragraph 264). Such interior exchange of love is eternal and perfect in unity – as well as invisible and therefore fully hidden from us but for God’s revealing it. This interior life and love of God is revealed to us in His works – beginning with creating and culminating in the “Incarnation,” or taking on of human flesh in the person of the Son (Jesus), and in the sending forth of the Holy Spirit. It is by way of this life of the Trinity in His works (or His “external” life, if you would) that we are able to know His interior life. In the next installment, we will begin to consider these external works of God – beginning with Creation itself.
For further reading: In the Catechism of the Catholic Church, paragraphs 232-267 speak to the basic teachings of God as Trinity, including how He reveals Himself and how the Church grew in her clarity of articulating this mystery through her earliest centuries. In the Scriptures, such passages as the baptism of Jesus, (Matthew 3:13-17; Mark 1:9-11; Luke 3:21-22; and John 1:31-34), or various speeches of Jesus (such as Matthew 11:25-27 or John 10:22ff and John 14:15ff) offer revelation of God as three distinct and unified persons of one and the same nature.