RESPONSE: The Holy Trinity is the union of the Father, Son and Holy Spirit as One God. As a fundamental truth of the Christian faith, no one embraces Christianity without believing in the Holy Trinity.
DISCUSSION: This central truth of our Christian faith is a mystery. Though Sacred Scripture contains many passages about the Trinity, nowhere in Sacred Scripture can we find a concise and thorough explanation of this mystery. Though we cannot understand fully the mystery of the Holy Trinity, we can explain, in a limited way, the characteristics of God as Trinity.
He Is Known by His Works
One way we come to an understanding of God is in terms of His work in creation as made known to us through divine revelation.
According to God’s revelation of Himself, the First Person of the Trinity, God the Father, created all things visible and invisible through His Word, the Son (Jn. 1:3). The Second Person of the Trinity, God the Son, became man in the person of Jesus Christ (Jn. 1:14; Col. 2:9). He died and rose to redeem mankind from sin and reconcile us to the Father (2 Cor. 5:14-21; Eph. 2:14-22; Col. 2:11-15). The third Person of the Trinity, God the Holy Spirit, is sent by the Father and the Son to form the Church (cf. Acts 2). Through the Church, He offers us the graces of salvation and sanctification. Thus, we sometimes speak of the Father as Creator, the Son as Redeemer, and the Holy Spirit as Sanctifier. The Father is revealed by His incarnate Son, Jesus, His perfect image. The Son reveals Himself by taking on our human nature. The Holy Spirit witnesses to the works of the Father and the Son, revealing Them both to man (cf. Catechism, nos. 238-246).
We thus speak of the Persons of the Trinity by how They relate to us as our Creator. We can also speak of Them by how They relate to each other.
From all eternity the First Person is Father, Who as He eternally knows Himself eternally generates the Son. The fundamental identity of the Second Person of the Trinity is as the Son, eternal generated by the Father, and as Son eternally loving Him in return. This Person has revealed Himself to man in the person of Jesus, Who is both God and man. Finally, the fundamental identity of the Third Person of the Trinity consists in His eternally being the Spirit of Love between the Father and the Son, a Love so real He is Himself a Person.
St. Bonaventure explains the relationship of the Three Divine Persons in terms of God’s absolute goodness. He states that to be the highest good, one must give himself (diffuse) in absolute fullness. This cannot occur unless the self-giving “is actual and intrinsic, substantial and hypostatic, natural and voluntary, free and necessary, lacking nothing and perfect.”  He continues:
Therefore, unless there were eternally in the highest good a production which is actual and consubstantial, and a hypostasis as noble as the producer, as in the case in a producing by way of generation and spiration, so that it is from an eternal principle eternally co-producing so that there would be a beloved and a co-beloved, the one generated and the other spirated, and this is the Father and Son and Holy Spirit—unless these were present, it would by no means be the highest good because it would not diffuse itself in the highest degree.
In these relations between the Persons of the Trinity, we can begin to glimpse the meaning of the words “God is love” (1 Jn. 4: 8). The human family mirrors this love of God when the love between husband and wife becomes so real it is a third person, a child.
Three Persons or Three Gods?
St. Patrick used the analogy of a shamrock to explain the Holy Trinity when converting the Celtic peoples. Just as in a shamrock there are three leaflets but one leaf, so in the Trinity there are Three Divine Persons but one God. When we ask the question, “What is God?” we are attempting to define His nature, which is God. When we ask “Who is God?” we are attempting to know the person and could get one of three answers: Father, Son or Holy Spirit. God is not God apart from the Three Divine Persons Who make up the Godhead, yet each of the divine persons completely possess the nature of God. They do not share the attributes of God in the sense of “taking turns”: one using perfect love, another using perfect mercy, etc. Nor are Father, Son, and Holy Spirit simply ways in which God shows Himself. All three Persons of the Trinity are distinct, eternal, and fully God.
How, then, does this not add up to three gods? The Father, Son, and Holy Spirit are not three separate gods because “the only real distinction between them lies solely in the relationships which relate them to one another” (Catechism, no. 155). “‘While they are called three persons in view of their relations, we believe in one nature or substance.’ ‘Indeed everything (in them) is one where there is no opposition of relationship’” (Catechism, no. 255, citing Councils of Toledo XI  and Florence ). Thus in God there is one divine substance (essence, nature), equally possessed by three distinct, divine Persons, whose distinction as Persons lies in the relationship of each to the others.
We often have difficulty with the idea of the Trinity because, as rational thinkers, we like to see everything as an equation. Three persons cannot equal one God. The equation does not add up, but God is not an equation and the Three Persons do equal one God. We struggle with this truth because the concept of three persons in one nature is quite beyond our experience. As St. Augustine pointed out, trying to contain this infinite mystery within our finite minds is like trying to pour the ocean into a seashell. We could not have arrived at this truth by human reason alone; it had to be revealed to us supernaturally by God.
In the Fullness of Time
Because the Trinity is such a deep mystery, God did not reveal this inner secret of His nature to man until man was ready to receive it. God is eternal, and all truth and history is eternally present to Him. We are limited by time, and only in time can we come to understand. In the Old Testament, when most peoples believed in many gods, the most important truth for the Hebrew people to understand was that there was only one God. After many centuries, “in the fullness of time, God sent His Son, born of a woman” (Gal. 4:4) and “sent the Spirit of His Son into our hearts, crying out ‘Abba, Father!’” (Gal. 4:6). Only after God’s people had the concept of one God firmly fixed in their minds were they ready to accept the truth of three persons in that one God, as Jesus Christ revealed it and the Holy Spirit taught it.
Even the early Church had to clarify the full truth of God’s revelation regarding His Triune nature. In the Councils of Nicaea (325) and Constantinople (381), the Church affirmed Christ’s full divinity as the incarnate Son of God. The Council of Constantinople also affirmed the divinity of the Holy Spirit. Thus in the Nicaean-Constantinopolitan Creed we profess:
We believe in one God, Father omnipotent, maker of heaven and earth, and of all things visible and invisible. And in one Lord Jesus Christ, the only begotten Son of God, born of the Father before all ages, light of light, true God of true God, begotten not made, consubstantial with the Father by whom all things were made…. And in the Holy Spirit, the Lord, the giver of life, who proceeds from the Father, who together with the Father and Son is worshipped and glorified.
Most misunderstandings of the nature of the Trinity either stress God’s unity at the expense of the distinction between Persons, or stress the distinction between Persons to the point that it seems there are three gods. Heresies that fall into the first extreme include Modalism, the idea that the three Persons of the Trinity are not in fact distinct Persons, but merely represent different “modes” or functions that the one true God performs. Modalists believed that God the Father really just “put on a robe of flesh” and became the person we know as Jesus Christ. Another misguided effort to defend that there is only one God is the belief that Jesus Christ is not the Second Person of the Trinity become incarnate—for the one God admits of no distinctions between persons—but rather a superior creature or a lesser divine/spiritual being. Arianism, a heresy condemned by the early church, was an example of this belief. Modern-day Jehovah’s Witnesses hold this belief as well. Heresies falling into the second extreme (believing that the Trinity is three gods—tritheism) are more rare, and would really be a form of that polytheism practiced by most primitive peoples. Mormons fall into this category, as they believe that Jesus and the Holy Spirit is fully divine but they do not recognize the unity of the Trinity as one God.
Unity of Persons, Unity of Man
Though God’s works reveal who He is (Catechism, no. 236), He cannot be reduced to His works or functions. This is why it is inappropriate to habitually refer to God as “Creator, Redeemer, and Sanctifier” rather than “Father, Son, and Holy Spirit.”
This unity of the Divine Persons provides the essential foundations of the Christian faith. Not only do we understand God within this unity of Persons, but we can better understand man, and God’s will for man. As noted above, the sacrament of Marriage is a natural expression of the unity of God. However, Jesus made very clear that Marriage, as we know it here on earth, does not exist in heaven (Mt. 22:29-30). Marriage on earth points to a greater union that God desires with us, a union with Him like His union with Himself. As said by Jesus, “The glory which Thou hast given me I have given to them, that they may be one even as we are one, I in them and Thou in me, that they may become perfectly one, so that the world may know that Thou hast sent me and hast loved them even as Thou hast loved me” (Jn. 17:22-23).
In addition to the sacrament of Marriage, we share this unity with God imperfectly here on earth through the celebration and reception of the other sacraments, particularly the Holy Eucharist. In Baptism we receive the Holy Spirit and become conformed to Christ as adopted sons of the Father. In Confirmation the graces of Baptism are completed and we are more perfectly conformed to Christ in the power of the Holy Spirit. In the most august sacrament of the Eucharist, we receive the Son intimately into our bodies as food for our journey here on earth. In the sacraments of Penance and Anointing we receive God’s mercy and healing as He strengthens our bond of unity with Him. In the sacrament of Orders, a man is configured to the person of Christ and ministers in persona Christi to the faithful here on earth. Thus, in our life as man, through true worship of Him in the sacraments, and through our daily work offered as a prayer to Him, we are expressions of God’s unity with Himself and we contribute to our union with Him in heaven.