The Church teaches that the Holy Trinity is the central mystery of the Christian Faith.
But how much do you know about this mystery?
What is its history?
What does it mean?
And how can it be proved?
Here are 12 things to know and share . . .

1. Where does the word “Trinity” come from?

It comes from the Latin word trinitas, which means “three” or “triad.” The Greek equivalent is triados.

2. When was it first used?

The first surviving use of the term (there may have been earlier uses that are now lost)  was around A.D. 170 by Theophilus of Antioch, who wrote:
In like manner also the three days which were before the luminaries, are types of the Trinity [Τριάδος], of God, and His Word, and His wisdom. And the fourth is the type of man, who needs light, that so there may be God, the Word, wisdom, man [To Autolycus 2:15].

3. What is the Trinity?

The Compendium of the Catechism of the Catholic Church explains it this way:
The Church expresses her trinitarian faith by professing a belief in the oneness of God in whom there are three Persons: Father, Son, and Holy Spirit.
The three divine Persons are only one God because each of them equally possesses the fullness of the one and indivisible divine nature.
They are really distinct from each other by reason of the relations which place them in correspondence to each other.
The Father generates the Son; the Son is generated by the Father; the Holy Spirit proceeds from the Father and the Son [CCC 48].

4. Is the Trinity the central mystery of the Christian Faith?

Yes. The Compendium explains:
The central mystery of Christian faith and life is the mystery of the Most Blessed Trinity.
Christians are baptized in the name of the Father and of the Son and of the Holy Spirit [CCCC 44].

5. When did the Church infallibly define the Trinity?

The dogma of the Trinity was defined in two stages, at the First Council of Nicaea (A.D. 325) and the First Council of Constantinope (A.D. 381).
First Nicaea defined the divinity of the Son and wrote the part of the Creed that deals with the Son.
This council was called to deal with the heresy known as Arianism, which claimed that the Son was a supernatural being but not God.
First Constantinople defined the divinity of the Holy Spirit and wrote the part of the Creed that deals with the Spirit.
This council dealt with a heresy known as Macedonianism (because its advocates were from Macedonia) which denied the divinity of the Holy Spirit. This heresy was also called Pneumatomachianism (from a Greek phrase meaning "fighting the Spirit").

6. How can the Trinity be proved?

The Trinity can only be proved through the divine revelation that Jesus brought us. It cannot be proved by natural reason or from the Old Testament alone. The Compendium of the Catechism of the Catholic Church states:
God has left some traces of his trinitarian being in creation and in the Old Testament but his inmost being as the Holy Trinity is a mystery which is inaccessible to reason alone or even to Israel’s faith before the Incarnation of the Son of God and the sending of the Holy Spirit.
This mystery was revealed by Jesus Christ and it is the source of all the other mysteries [CCC 45].
Although the vocabulary used to express the doctrine of the Trinity took time to develop, we can demonstrate the different aspects of the doctrine from Scripture.

7. How can we show from Scripture that there is only one God?

The fact that there is only one God was already made clear in the Old Testament. For example, the book of Isaiah proclaims:
“You are my witnesses,” says the Lord, “and my servant whom I have chosen, that you may know and believe me and understand that I am He.  Before me no god was formed, nor shall there be any after me [Isaiah 43:10].
Thus says the Lord, the King of Israel and his Redeemer, the Lord of hosts: “I am the first and I am the last; besides me there is no god" [Isaiah 44:6].

8. How can we show that the Father is God? 

The Father is proclaimed as God numerous times in the New Testament. For example, St. Paul declares:
Blessed be the God and Father of our Lord Jesus Christ, the Father of mercies and God of all comfort [2 Corinthians 1:3].
There is  . . . one God and Father of us all, who is above all and through all and in all [Ephesians 4:4-6].

9. How can we show that the Son is God?

This is proclaimed in a variety of places in the New Testament, including at the beginning of the Gospel of John:
In the beginning was the Word, and the Word was with God, and the Word was God.
And the Word became flesh and dwelt among us, full of grace and truth; we have beheld his glory, glory as of the only Son from the Father [Jn. 1:1, 14].
And later:
Then [Jesus] said to Thomas, “Put your finger here, and see my hands; and put out your hand, and place it in my side; do not be faithless, but believing.”
 Thomas answered him, “My Lord and my God!” [Jn. 20:27-28].

10. How can we show that the Holy Spirit is God?

In the book of Acts, the Holy Spirit is portrayed as a divine Person who speaks and who can be lied to:
While they were worshiping the Lord and fasting, the Holy Spirit said, “Set apart for me Barnabas and Saul for the work to which I have called them” [Acts 13:2].
But Peter said, “Ananias, why has Satan filled your heart to lie to the Holy Spirit and to keep back part of the proceeds of the land? . . . You have not lied to men but to God” [Acts 5:3-4].

11. How can we show that the Father, the Son, and the Spirit are distinct Persons?

The distinction of the persons can be shown, for example, in the fact that Jesus speaks to his Father. This would make no sense if they were one and the same person.
At that time Jesus declared, "I thank thee, Father, Lord of heaven and earth, that thou hast hidden these things from the wise and understanding and revealed them to babes; yea, Father, for such was thy gracious will" [Matthew 11:25-26].
The fact that Jesus is not the same Person as the Holy Spirit is revealed when Jesus--who has been functioning as the Counselor (Greek, Parakletos) of the disciples--says he will pray to the Father and the Father will given then "another Counselor," who is the Holy Spirit. This shows the distinction of all three Persons: Jesus who prays; the Father who sends; and the Spirit who comes:
And I will pray the Father, and he will give you another Counselor, to be with you for ever, even the Spirit of truth, whom the world cannot receive, because it neither sees him nor knows him; you know him, for he dwells with you, and will be in you [John 14:16-17].

12. How can we show that the Son is generated by the Father and that the Spirit proceeds from the Father and the Son?

The fact that the Son is generated by the Father is indicated by the names of these Persons. Sons are generated by fathers. The Second Person of the Trinity would not be a Son if he were not generated by the First Person as his Father.
The fact that the Holy Spirit proceeds from the Father and the Son is reflected in another statement of Jesus:
But when the Counselor comes, whom I shall send to you from the Father, even the Spirit of truth, who proceeds from the Father, he will bear witness to me [John 15:26].
This depicts the Holy Spirit proceeding from the Father and the Son ("whom I shall send"). Here the outward operations of the Persons of the Trinity reflect their mutual relations with each other. It may also be said that the Holy Spirit proceeds from the Father through the Son.