MC Web Application
Page Links

Hear, O Israel: The LORD our God is one LORD. (Dueteronomy 6:4)

Then God said: "Let us make man in our image, after our likeness. Let them have dominion over the fish of the sea, the birds of the air, and the cattle, and over all the wild animals and all the creatures that crawl on the ground." God created man in his image; in the divine image he created him; male and female he created them.
(Genesis 1:26-27; of many OT verses, see also Psalm 2:7, Psalm 110:1, Isaiah 6:8, 61:1)

In the beginning was the Word, and the Word was with God, and the Word was God. (John 1:1)

[Jesus speaking] "The Father and I are One" (John 10:30)

[Jesus speaking] "And I will ask the Father, and he will give you another Advocate [the Holy Spirit] to be with you always, the Spirit of Truth" (John 15: 16-17)

Then Jesus approached and said to them, "All power in heaven and on earth has been given to me. Go, therefore, and make disciples of all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit, teaching them to observe all that I have commanded you. And behold, I am with you always, until the end of the age."
(Matthew 28:18-20)

God is Love, and he who abides in love abides in God, and God abides in him.
(1 John 4:16).

If you see charity, you see the Trinity.
(St. Augustine, De Trinitate, VIII, 8, 12: CCL 50, 287)


The doctrine of the Blessed Trinity is the central doctrine of the Catholic faith. No one can profess to be Catholic (or Christian in general) without believing in this essential doctrine, defined as follows: the doctrine of the Blessed Trinity is belief in one God in three divine Persons -- Father, Son, and Holy Spirit. Catholics believe in the Trinity because it was revealed to us by God, most definitively in the person of Jesus Christ. As Session 1 has shown, human reason can give us some knowledge of God including good reasons to believe in God's existence and some knowledge of God's nature and attributes. However, the doctrine of the Trinity is not something human reason could ever have discovered on its own, and thus so that we might fully come to know, love, and serve God with all our minds and hearts, God revealed to us his innermost life, which is love: the Father, Son, and Holy Spirit giving each to the other in an eternal communion of self-donating love. St. John tells us that God is Love (see 1 John 4:16 above), and the Good News that Jesus Christ came to share with us is that we were created by the God who is Love to share in the life and love of the Trinity forever. Below we will examine this central doctrine of the Catholic faith more closely.

How do we come to know God?

In Session 1 we saw that using our natural human reason we can come to know certain things about God and his nature. Namely, that He exists and is one God, is spiritual, all powerful, eternal, infinite, and a personal Being. But the most essential inner-life of God we never could have discovered using our reason, and so God revealed to us that He is a Trinity, or Tri-Unity of persons. God has revealed to us that He is one God in three divine Persons. This means that there is only one divine being or substance that is God, which is fully expressed in the three Persons of the Trinity: God the Father, God the Son, and God the Holy Spirit. All three persons form a unity of the Godhead but are also distinct; meaning that all three persons are one God but the Father is not the Son, and the Son is not the Holy Spirit, and the Holy Spirit is not the Father.

All of salvation history (recorded in the Bible in the Old and New Testaments) is a story of God's love for us. The essence of that story is that God is Love (see 1 John 4:16 above), and that out of love He created us to share his life forever. When scripture reveals to us that God is Love, what does that mean? It means exactly what the doctrine of the Trinity expresses, that God in his one divine being is a loving communion (i.e., common union) of persons. The Father from all eternity generates and loves the Son, and the Son from all eternity receives and returns the Father's love, and the love between Father and Son from all eternity is so real that it IS the third person, the spirit of love between Father and Son, the Holy Spirit.

God gave us intimations of his innermost Trinitarian life in the Old Testament. For instance, in the quote from Genesis listed above God said "let us make man in our image", using the plural words "us" and "our" to refer to Himself. This plurality Christians believe indicate the Triune Godhead. The book of the Psalms and the book of the Prophet Isaiah also contain veiled references to the Trinity (see Old Testament quotes above). But the people of the old covenants were not ready to receive the full revelation of God's nature, and surrounded by polytheistic cultures God had to first reveal to his people that there is only one God (see Dt. 6:4 above), and that the one God is the God of Israel. But in the fullness of time, Jesus Christ, the 2nd person of the Trinity, assumed a human nature and entered time and history "for us men and for all salvation" to reveal God's life and love fully to men. Salvation history culminated in God entering history as one of us in order to share God's innermost secret with us his children, and that is we are made in the image of the Triune God who is Love itself. There are many passages in the New Testament where Jesus Christ reveals the Trinitarian nature of God whole or in part. But one quote (also listed above) contains the doctrine explicitly, where Jesus after his resurrection from the dead (but before ascending into heaven) gives his apostles the grand commission: "All power in heaven and on earth has been given to me. Go, therefore, and make disciples of all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit, teaching them to observe all that I have commanded you. And behold, I am with you always, until the end of the age." We see here that Jesus Christ commands his apostles to teach and baptize with the authority given to him by his Father, and He explicitly tells them to baptize in the name, not names, of the Father, Son, and Holy Spirit. The Trinity has only one Name because there is only one God, and this is why Jesus used the singular word "name" instead of the plural "names". But Christ reveals further that the one God is fully expressed in three divine persons by commanding his apostles to baptize in the name of the Father, Son, and Holy Spirit.

The Mystery of the Trinity

What is mystery? In common use, a mystery refers mainly to a type of puzzle that has a solution, as in a murder mystery. When applied to the doctrines of the faith, such as the Trinity or the Incarnation, mystery takes on an entirely new meaning. A mystery of the faith is defined as a reality that cannot be fully comprehended. Accordingly, mystery when used in a faith context refers to something REAL, which although unable to be fully comprehended is yet knowable to some degree.

As God is the ultimate reality behind all the mysteries of the faith, our inability to fully comprehend the mysteries makes sense if you recall something from Session 1, which is that God is infinite: He has no limit or lack, He is perfect. Since God is infinite, and we his human creatures are finite, it remains forever impossible for the finite to ever come to fully know the infinite. [see Appendix 1]. Because God is infinite and we are finite, it is essential to remember that human language and concepts can never truly capture the reality of God, including his inner life as Trinity. But since we can only use what we have, the Church with the help of the Holy Spirit has for 2000 used human language to define and defend the doctrine of the Trinity against all false teachers (i.e. heretics). The precise doctrinal formulations hammered out by the Church are part of the Church's tradition, and are encapsulated in the creeds that we profess as Catholics. The traditional heresies related to the Trinity fall into generally two categories: 1. those heresies that deny that there is one God in three persons and assert rather that there is one God and that Father, Son, and Holy Spirit are just different aspects or modes of the one God's existence (i.e. Modalism); 2. those heresies that deny that there is one God in three persons and assert rather that the Father, Son, and Holy Spirit are really three Gods (i.e. Tritheism).

The doctrine of the Blessed Trinity, the central doctrine of the Catholic faith, although a mystery has come to be known more fully over the last two millennia of the Church's history. The Nicene Creed, which is professed at every Sunday Mass, contains the Church's formulation of the doctrine which provides a wonderful summary (and defense) of our Trinitarian Catholic faith.

Is there a Contradiction?

Although human reason could never have discovered the Trinity, we can use human reason to show that this mystery does not contradict reason, although it surpasses it. For instance, we can reason that there is no contradiction in professing one God in three divine persons since we are not asserting that one person equals three persons, or one god equals three gods. [see Appendix 2 for some analogies of three in one]. A quick look at the meanings of the words "person" and "nature" in this context can help clarify why this is so.

The difference between nature and person is that a nature (as in a human nature) shows "what" a thing is, and a person "who" someone is. We can describe what a being is like by describing its nature (e.g. rock nature, feline nature, human nature, angelic nature, divine nature etc...), but to know Who someone is, we must know the person. To be a person, a being must possess a rational nature (i.e., having the powers of intellect and will). In short, persons (the who) act according to their natures (the what).

When we ask the question, "What is God?" we are attempting to define His nature, which is the divine nature or God (from Session 1 we learned about God's nature which is one, spiritual, all powerful, all-knowing, eternal, infinite, perfect). When we ask "Who is God?" we are attempting to know the person and could get one of three answers based on God's revelation to us: 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 possesses 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 (remember the Modalist heresy above). All three persons of the Trinity are distinct, eternal, and fully God. How, then, does this not add up to three gods (remember the Trithiesitc heresy above)? 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 of the Catholic Church [CCC], no. 155). " 'While they are called three persons in view of their relations, we believe in one nature or substance.' (CCC, no. 255). 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. The Father eternally generates or begets the Son, the Son is eternally generated or begotten by the Father, and the Holy Spirit proceeds from Father and Son as the bond of love between them, a bond so real that it IS a person.

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 famously pointed out, trying to contain this infinite mystery within our finite minds is like trying to pour the ocean into a seashell. As stated before, we could not have arrived at the truth of God's inner-life by human reason alone; it had to be revealed to us by God himself.

The doctrine of the Trinity is a great mystery, and thus it should not trouble us that we have difficulty comprehending God in his innermost being since we are finite and conditioned to see things in a finite way, but God is infinite and totally beyond our full reach and comprehension. The most important thing to remember is that God revealed to us that He is one God consisting of a Trinity or Tri-Unity of divine persons. God revealed his inner-life to us not to confuse us, but because out of love for us He always wants to share more of Himself with us, not less. And what God shares with us is the reality that He is Love, and that we were created to share in the love of the Trinity forever.

Uniqueness of Christianity: God is Love

Christianity is the only faith that can say God is Love and have that meaning concretely realized in the Godhead itself. The Catholic philosopher Peter Kreeft puts it this way: "If God is not a Trinity, God is not love. For love requires three things: a lover, a beloved, and a relationship between them. If God were only one person, he could be a lover, but not Love itself. The Father loves the Son and the Son loves the Father, and the Spirit is the love proceeding from both, from all eternity. If that were not so, then God would need us, would be incomplete without us, without someone to love. Then his creating us would not be wholly unselfish, but selfish, from his own need".

The doctrine of the Trinity means that relationship is the fundamental category of reality, because it goes all the way up into ultimate reality, into God. God is a society, a family. God is love. Because we are made in the image of the Trinity, love and family, friendship and community are not accidental but essential to us, at the very heart of human existence. The previous Pope John Paul II delivered a profound teaching referred to as his Theology of Body on this subject. The Holy Father taught that since man is created in God's image, even his very body points to the central mystery of God's inner life. From Genesis 1:26 quoted above, we see that God created us in his image, as male and female. The complementarity of the male and female bodies reveals that man and women are made for each other, made for loving communion in which each spouse makes a total gift of self to the other. And that love between the spouses can issue forth new life, creating a family. This communion of human persons reflects the love and unity of the Trinity, in whose image we are made. And so Pope John Paul teaches based on God's revelation to us which includes even our bodies that our true vocation is for self-giving love: love of God and of neighbor.

To sum up, the doctrine of the Blessed Trinity ultimately expresses the reality that God in his innermost being is a loving communion of persons, in other words a Family. And our great destiny as creatures made in God's image, is to share in the life and love of that eternal Family forever. The Family of the Trinity then by God's invitation becomes our Family; we enter it here on earth through the Church by faith and baptism, where we receive the Spirit of adoption and become through this adoption the sons and daughters of our heavenly Father, co-heirs to eternal life through our Lord and Savior Jesus Christ (Rom 8: 15-17). As the old Baltimore Catechism states, "God created us to know Him, love Him, and serve Him in this world and to be happy with Him forever in Heaven." Our eternal destiny is to become part of God's Family through the Catholic Church founded by Christ for "us men and for our salvation". And once we are united to Christ in his Mystical Body, which is the Church, we can begin to participate in the life of the Family of God now and forever, and no greater Family reunion can be imagined. As scripture tells us, "eye has not seen, and ear has not heard ... what God has prepared for those who love him". (1 Corinthians 2:9).


We briefly looked at the central doctrine of the Catholic faith, which is the doctrine of the Blessed Trinity. This essential truth of God's inner life, we saw, was hinted at in the Old Testament and only revealed to us in the fullness of time by the second person of the Trinity who became man for our sake, Jesus Christ. We discovered that this doctrine is a mystery, a reality that cannot be fully comprehended. But this is a reason for rejoicing, as we can enter into a relationship with God and NEVER exhaust the joy, knowledge, and peace that comes from sharing the infinite life and love of the Trinity. We have seen that all people are called to become part of God's Family, the Trinity, forever.

Additional resources on the Blessed Trinity:

See printable version of this document in word format
Catechism of the Catholic Church (CCC) paragraphs 232-267
Faith Facts
Frank Sheed (Catholic theologian) on the Trinity
John A. Hardon, S.J, on the Doctrine of the Trinity
Peter Kreeft (Catholic philosopher) on LOVE

Appendix 1: Analogy on Finite not comprehending the Infinite

To illustrate the impossibility of the finite human mind fully comprehending God, who is infinite and perfect, here is a short, crude analogy. Imagine several two dimensional (length and width) squares drawn on a sheet of paper. Also, imagine that all the squares have consciousness (i.e. can think about themselves and their 2 dimensional world) and one square told another square about an object existing in 3 dimensions (length and width and depth) called a Cube that has revealed itself to the 2 dimensional squares. Imagine the first square trying to make sense of and comprehend what the Cube is like existing in 3 dimensional space. The Cube has much more to it than a square can comprehend (i.e., the extra dimension), and although it can understand something about the Cube because the square has a likeness to the Cube, sharing the property of "squareness", the deeper knowledge concerning the extra dimension of the Cube and all the possibilities of 3 dimensional space must be revealed from outside the 2 dimensional universe of the square. And even with explicit revelation of these 3 dimensions from the Cube itself, the 2 dimensional square will never fully comprehend the Cube and its 3 dimensional universe. So it is with the infinite God who reveals Himself to our finite minds. If this analogy is not helpful, disregard it immediately.

Appendix 2: Analogies for the Trinity

Included here briefly are some analogies that may shed some light on this central mystery of the Trinity. An analogy is a comparison based on the similarities between two things. When applied to God, however, any similarity is always outweighed by an even greater dissimilarity. This is so because, as was said above, the infinite (i.e., God) can never be adequately compared to or comprehended by the finite (i.e., us). So if these analogies are not helpful, as mentioned in Appendix 1 disregard them immediately.

Shamrock: St. Patrick is alleged to have used the shamrock plant to help the Celtic inhabitants of Ireland understand the Trinity. A shamrock is one plant, with three leaves. The analogy here is to the one God (the one plant), existing in three divine persons (the three leaves).

Triangle: A triangle is a geometrical figure with 3 internal angles the sum of which is 180 degrees. The triangle is one thing (a geometric figure), but is comprised of 3 distinct things (the angles). The analogy here is to the one God (the triangle) existing in three divine persons (the 3 angles).

Analogy of Mind or Soul: St. Augustine sees an image or analogy of the Trinity in the human soul's three powers of memory (which transcends time like the Father), understanding (which is like the Word of God who is the Son, see John 1:1 quoted above), and will (which is like the Holy Spirit, the spirit of love between Father and Son, for love is the work of the will).