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.

— Matthew 28:19

Either before you arrive or as you wait for class to begin, find this short passage from Sacred Scripture in your Bible or through an online search. Reading it will prepare you for this week’s material.

—John 14:2–10, 15–18, 23, 25–26

God our Father, who by sending into the world the Word of truth and the Spirit of sanctification, made known to the human race your wondrous mystery, grant us, we pray, that in professing the true Faith, we may acknowledge the Trinity of eternal glory and adore your Unity, powerful in majesty. Through our Lord Jesus Christ, your Son, who lives and reigns with you in the unity of the Holy Spirit, one God, for ever and ever. Amen.

As you watch the video lesson, refer to these key highlights. Feel free to use the space provided to take notes.

• Belief in the Trinity is the central claim of Christianity.

• There are two main mistakes we can make regarding the Trinity: to think that the three persons are separate from each other or to think that they really aren’t distinct from each other.

• St. Thomas Aquinas built on St. Augustine’s insights by using an analogy for the Trinity where he describes the process of thinking, or imagining, one’s own self in the mind.

• The Word of God proceeds by way of intellect, and the Holy Spirit by way of love.

• Christians worship God in a Trinitarian form, including the words we use.

Wisdom of the Saints

O Eternal Word, utterance of my God, I want to spend my life listening to you, to become totally teachable so that I might learn all from you. . . . O Consuming Fire, Spirit of Love, overshadow me so that the Word may be, as it were incarnate again in my soul. . . . And you, O Father, bend down towards your poor little creature. Cover her with your shadow, see in her only your beloved son in who you are well pleased.

— St. Elizabeth of the Trinity

Take a moment to go through these questions with the group. For larger classes, divide them up into partners. Answers are provided to assist you in leading discussion.

1. Why is belief in the Trinity so important for Christians?
   We are baptized in the one name of the Triune God, and we worship “in the name of the Father, and of the Son, and of the Holy Spirit.” God wants us to know him as he really is, to know him intimately and personally, as Father, Son, and Holy Spirit. If you truly understand who Jesus is—the Son of God sent by the Father into the world to become man, and who sends us the Holy Spirit—then it is necessary to say that God is Father, Son, and Holy Spirit.

2. How do the names “Father” and “Son” help us understand the mystery of the Trinity?
   “Father” and “Son” are names that point to a mutual relation; they imply each other. Even among human beings, a man is never a father unless there is a child. In the Trinity, the Divine Persons are relative to each other.

1. Are the Divine Persons really different beings, or different gods?

In response to Arianism, the Council of Nicaea (AD 325) taught that the Divine Persons are consubstantial (of the very same substance or being). They are distinct, but each person is fully and entirely God, co-equal and co-eternal with the others. Each person is “in” the other; they can only be distinguished from each other based on their relations to each other: the Son proceeds “from” the Father, and the Holy Spirit proceeds “from” the Father and the Son. They are not three different beings or three different gods, but one single God in three persons.

1. How does the analogy of forming a “word” in your mind help to understand the relation between the Father and his eternal Word?

In one eternal “now,” God understands himself in an infinitely perfect way. He conceives an “image” of himself; that is, a single divine “Word” that expresses himself absolutely perfectly. This spiritual and immaterial “Word” is perfectly identical with the one who “speaks” it in eternity in absolutely every way except for one: the divine Word is God “from” God.

Catechism Connection

The mystery of the Most Holy Trinity is the central mystery of Christian faith and life. It is the mystery of God in himself. It is therefore the source of all the other mysteries of faith, the light that enlightens them. It is the most fundamental and essential teaching in the “hierarchy of the 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 sin.”

—CCC 234

Discuss these questions with the group, pair them with a partner, or ask them to meditate on their own.

1. How important is it for you to understand something about who God is, in person? How might this help you in your prayer life?

2. What might you say to someone who claimed that God cannot be a Trinity?

3. What is the significance for you that your Christian life begins by being baptized in the name of the Father, and of the Son, and of the Holy Spirit?

After concluding your group discussion, return to watch the second video where converts to the Catholic faith discuss their conversion and how they live today as Catholics.

Rome to Home

If anyone will not confess that the Father, Son, and Holy Spirit have one nature or substance, that they have one power and authority, that there is a consubstantial Trinity, one Deity to be adored in three subsistences [Greek: hypostaseis, Latin: subsistentiae] or persons, let him be anathema. For there is only one God and Father, from whom all things come, and one Lord, Jesus Christ, through whom all things are, and one Holy Spirit, in whom all things are.

—Second Council of Constantinople, confirmed by Pope Vigilius in AD 553

Trinity: The mystery of one God in three Persons: Father, Son, and Holy Spirit.

Divine Person: The term used to describe the Father, Son, and Holy Spirit in their real relation to and distinction from one another within the unity of the Blessed Trinity. Each of the three Divine Persons is fully God and is absolutely equal to the other two. Each Person is a “subsistence,” or a subsisting relation in the one divine nature (see CCC 252).

Divine Nature: The divinity or the divine essence. The one divine substance. Each of the three distinct persons of the Trinity is entirely God, who is one by the divine nature (see CCC 253).

Consubstantial: Of the very same substance or being. This term is used in the Nicene Creed in order to emphasize that the Divine Persons are of the very same, single divine substance or being. It underlines the oneness and unity of the Divine Persons. (This term was originally chosen to correct the heretical view of Arianism, which claimed that the Son was a separate being than the Father, and thus, not truly God.)

Arianism: The ancient heresy, associated with a priest named Arius, which taught that the Son was not eternal like the Father, but rather was only begotten by the Father in time. On this view, the Son was a separate being than the Father, and thus, less than the Father and not truly God. Arius claimed that the Son was not “consubstantial” with the Father.

Sabellianism: The ancient heresy, associated with a theologian named Sabellius, which claimed that the distinctions between the Divine Persons were not really “in” God but were only how the one God appears to us as he acts in the world. On this view, the Divine Persons are not real or really existing in God, they are just “appearances” as we look at God from our perspective.

God’s Word

There is one body and one Spirit, just as you were called to the one hope that belongs to your call, one Lord, one faith, one baptism, one God and Father of us all, who is above all and through all and in all.

— Ephesians 4:4-6

Leader: Our Help is in the name of the Lord.

Response: Who made heaven and earth.

All make the sign of the cross.

This serves as a review of the material that your students can read after each class, but it may be helpful for you to read as well. Consider reading it before each class to better prepare you for group discussion.

At every Sunday Mass, we profess that we believe in the Holy Trinity, one God, who is Father, Son, and Holy Spirit. Indeed, this is the central claim of Christianity. Every Christian is baptized in the one name of the Triune God (see CCC 233), and every Mass begins “in the name of the Father, and of the Son, and of the Holy Spirit.” Believing in the Trinity is the very substance of being a Christian, consecrated by baptism in this holy and mysterious name.

The Holy Trinity is the mystery of God in himself: God wants us to know him as he really is, to know him intimately and personally, as Father, Son and Holy Spirit—the one God who is a perfect and infinite communion of life and love of three Divine Persons. This is, in a sense, the highest and greatest truth we know about God. We could not discover it by reason alone. This doesn’t mean that it is unreasonable or irrational—we don’t believe in a contradiction—but we only know it by the light of faith because God has revealed it.

The one who reveals this mystery is, above all, Jesus Christ. In fact, if you truly understand who Jesus is—the Son of God sent by the Father into the world to become man and who sends us the Holy Spirit—then it is necessary to say that God is Father, Son, and Holy Spirit. As Pope Leo XIII wrote, the mystery of the Holy Trinity is “the substance of the New Testament.”

Ancient Israel professed belief in one God. “I am the LORD, and there is no other, / besides me there is no God” (Is 45:5). Throughout the centuries, the Jewish people were persecuted for refusing to acknowledge the pagan gods of other nations. St. Paul professes the same truth: “God is one” (Rom 3:30).

Yet Scripture also reveals that Jesus is God the Son, sent by the Father. St. Paul says that every tongue will confess Jesus Christ as LORD, to the glory of God the Father, using the divine name (“LORD”) reserved in the Old Testament for the one God of Israel (see Phil 2:11). Jesus often spoke and preached about his Father as distinct from himself: “I came from the Father and have come into the world” (Jn 16:28). He also says that he is in the Father, and the Father is in him. “I and the Father are one” (Jn 10:30). “All things have been delivered to me by my Father; and no one knows the Son except the Father, and no one knows the Father except the Son and any one to whom the Son chooses to reveal him” (Mt 11:27). “No one comes to the Father, but by me” (Jn 14:6). Likewise, he tells us that he will send the Holy Spirit, who is the Spirit of Truth, from the Father (see Jn 14:16–17). We also read that the Holy Spirit is the Spirit of the Son; the Spirit gives us a share in the eternal life that is Christ’s heritage and pours the love of God into our hearts (see Rom 8:9; Gal 4:6; Rom 5:5), by whom we profess “Jesus is LORD.”

During her first centuries, the Church sought to clarify her Trinitarian faith to both deepen understanding and defend it from errors. We do not believe in three Gods but one God in three persons. Each Divine Person is fully God, whole and entire; they are not “parts” of God. And each person is equal to the others and distinct from them (though never separated from them).

The names of the Divine Persons (like “Father” and “Son”) help us understand this mystery. In a human family with a single male child, the father becomes a father only at the moment when the son becomes a son. These names point to a mutual relation. They imply each other. And in the Trinity, the Divine Persons are relative to each other; in fact, the Divine Persons are relations subsisting in the one divine nature.

Scripture also reveals that the Son is the divine “Word” (Jn 1:1). Building on an insight of St. Augustine, St. Thomas Aquinas develops what many consider to be the best analogy for understanding how the Trinity is one God in three Persons.

Try this thought experiment: Think of yourself. Imagine being able to conceive, in your mind, a perfect image of your own mind, or even a mental image of your very self, that perfectly captures who you are, everything about yourself.

For us, forming a perfect “idea” of ourselves is very difficult. Our minds are limited, and we don’t perfectly understand ourselves. We are creatures in time: we have bodies, we change, we have a past and a future, and much remains obscure and unknown to us. But this thought experiment shows at least this: as you (imperfectly) understand yourself, there is something in your mind, a kind of “idea” or “image” or mental “word”, that is both “from” you and remains “in” you, distinct from you who are thinking it, but that also corresponds in some (imperfect) way to who you are. It is the fruit of your act of understanding yourself. In a very limited and partial way, your own mind can be “in” your mind without producing another “you.”

Maybe by now you are a little dizzy! But keep reading. It’s important to stretch our minds when contemplating the great mysteries of God. He will surely reward our efforts!

We know that God’s intellect is infinitely superior to ours. In one eternal “now,” he understands himself in an infinitely perfect way. He conceives an “image” of himself; that is, a single divine “Word” that expresses himself with absolute perfection. This is the Son, “the image of the invisible God” (Col 1:15). Since God is a pure spirit without a body, his spiritual and immaterial “Word” is perfectly identical with the one who “speaks” it in eternity in absolutely every way except for one: the divine Word is God “from” God. This is the eternal “generation” or “begetting” of the divine Son. “In the beginning was the Word, and the Word was with God, and the Word was God” (Jn 1:1). And as God conceives his Word by one perfect act of self-understanding, he loves what he understands as perfectly and infinitely good, and so from the Father and the Son proceeds a third Person: the Holy Spirit, who is divine love “in person.” To put it another way, along with the Father, the Father’s Word breathes forth Love in the person of the Holy Spirit.

There are thus two “processions” in God: the procession of the Word from the Father by way of intellect or understanding and the procession of the Holy Spirit from the Father and the Son by way of love. This analogy helps us understand how there can be distinct Persons “in” the one God who simply are one God, one divine nature. The Father’s Word is absolutely identical to the Father, “consubstantial” or “one” with the Father. He is “in” the Father, and the Father is “in” him. He only differs from the Father in one respect: he is “from” the Father. The Holy Spirit is also identical to the Father and the Son in every way except for one: he is “from” the Father and the Son, who breathe him forth by way of love.

In the Church’s early centuries, she had to confront wrong understandings of what Scripture teaches about the Father, Son, and Holy Spirit. Two dangerous errors stand out: Arianism and Sabellianism.

In AD 318, a priest named Arius began to preach that the Son was not “true God” like the Father but was rather the highest and first creature that the Father created, above the angels but less than truly divine. He denied that the Son was “consubstantial” (Greek: homoousios) with the Father and said that there was a time before the Son existed. In other words, he thought that the Father and Son were not just distinct persons but really different beings and only the Father was truly God. This heresy was condemned by the Council of Nicaea in AD 325 as incompatible with Scripture. That council produced the creed we say at Mass in order explicitly to rule out what Arius taught: “I believe in one God, the Father almighty . . . and in . . . the Only Begotten Son of God, born of the Father before all ages, God from God, light from light, true God from true God, begotten, not made, consubstantial with the Father.”

The other great heresy goes to the opposite extreme, denying that there is any real distinction between the Father and the Son. Around AD 220, Pope Callistus I excommunicated a theologian named Sabellius, who claimed that God the Father appeared on earth as the Son: the Son is really the Father. For Sabellius, the persons are not really distinct, they only look distinct to us as we see them active in the world. Sabellianism says: “when God creates, we call him the Father; when he redeems us, we call him the Son; when he sanctifies us, we call him the Holy Spirit—but these distinctions are not really in God, but are just different ways or modes in which the one God appears to us.” Like Arianism, this heresy denies a central truth of the Christian faith: we believe that Father, Son, and Holy Spirit are really distinct persons in the one God. We profess that the Son was sent by the Father, became man, and suffered on the Cross. The Father did not become incarnate and was not crucified.

The most basic truth about the Trinity is that God is a communion of persons who is perfectly happy in himself. He does not need us. But he so loved the world that he sent his only begotten Son to suffer on the Cross and sent his Holy Spirit into our hearts in order to draw us into the mystery of his triune life so that we, too, could share in the mystery of the infinite love and the perfect life of the Triune God.

Emery, Gilles. The Trinity: An Introduction to Catholic Doctrine on the Triune God. Translated by Matthew Levering. Washington, DC: The Catholic University of America Press, 2011.