A Brief Introduction to the Blessed Trinity

Trinity in Scripture:

[Note that this is a very superficial treatment of what we see of the Trinity in Sacred Scripture.  See the list of further reading below for links to much more detailed treatements of this matter.]

Matthew 28:19 - "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,"

When demonstrating the Trinity there are four points to establish:

1. The Father is God
1 Corinthians 1:3 - "Grace to you and peace from God our Father and the Lord Jesus Christ."

2.  The Son is God
John 20:28 - "Thomas answered him, "My Lord and my God!""
Hebrews 1:8 - "But of the Son he says, "Thy throne, O God, is for ever and ever, the righteous scepter is the scepter of thy kingdom."

3.  The Holy Spirit is God
Acts 5:3-4 - "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? 4 While it remained unsold, did it not remain your own? And after it was sold, was it not at your disposal? How is it that you have contrived this deed in your heart? You have not lied to men but to God.'"
2 Corinthians 3:17-18 - "Now the Lord is the Spirit, and where the Spirit of the Lord is, there is freedom. 18 And we all, with unveiled face, beholding the glory of the Lord, are being changed into his likeness from one degree of glory to another; for this comes from the Lord who is the Spirit."

4.  There are not three Gods, but one God
Deuteronomy 6:4-5 - "Hear, O Israel: The Lord our God is one Lord; 5 and you shall love the Lord your God with all your heart, and with all your soul, and with all your might." (Note that in the NT, Jesus says the same thing in Mark 12:29).
Isaiah 43:10-11 - "'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. 11 I, I am the Lord, and besides me there is no savior.'"

For a bit more about Jesus and his role in the Godhead, have a look at these passages:
Philippians 2:5-11; Colossians 1:15-20; John 17:3-5.

How Can We Explain the Trinity?
It's a mystery, the greatest mystery of our faith, but that doesn't mean we can't explore it.  Some people think a mystery of the faith is like a museum with a big brick wall in front of it so that we will never know what is inside.  Really it is like a museum that we are welcome to wander around in as much as we like, but there are an infinite number of exhibits so we will never finish learning what there is to know.  It is important that we make the effort to have a look around though, because it is important to know God as he is.  That's what we will be doing in heaven (1 John 3:2 - "Beloved, we are God’s children now; it does not yet appear what we shall be, but we know that when he appears we shall be like him, for we shall see him as he is").

If we want to get close to God we should try to know him personally, not just by what he does (Creator, Redeemer, Sanctifier is one way to put it), but by who he is (Father, Son and Holy Spirit).

Here are the basics of the doctrine of the Trinity:
1. In the one divine nature, there are three persons, the Father, the Son, and the Holy Spirit.
2. No one of the persons is either of the others, each is wholly himself.
3. The Father is God, the Son is God, the Holy Spirit is God.
4. They are not three Gods but one God.

Some Background - God is Existence
A key truth about God's nature is that He is existence.  Everything in the universe is dependent upon something else for its existence.  We each have the possibility of not existing.  But for anything to exist at all, there must be something that is not dependent on something else for existence.  This something is God.  God cannot not-exist; God is existence.  Figuring this out was one of the greatest achievements of the Greek philosophers of the 5th century BC.  But a good thousand years before that, the Jews knew it too - but only because God told them:
God said to Moses, "I AM WHO I AM." And he said, "Say this to the people of Israel, ‘I AM has sent me to you.’" (Ex 3:14).

Some More Background: Person and Nature
We have to be able to distinguish between person and nature.  Put simply, our nature determines what we are; person describes who we are.

The Trinity is a Revealed Doctrine
God is a pure spirit, and a better understanding of the Trinity comes from thinking about the fact that it is God's nature to exist, and about the actions of spirit: knowing and loving.  We should also remember that the inner life of God is not something we could have figured out for ourselves apart from Revelation.

The Second Person
Perhaps the biggest clue is found at the start of John's Gospel: "In the beginning was the Word, and the Word was with God, and the Word was God".  Now, God is pure spirit, so a word produced by God would be more like a thought or an idea.  So what idea produced in God's mind could possibly be God?  The idea that God has of himself.

When God thinks of himself, this thought must be perfect, because God is perfect, so whatever is in the Father must be in this idea of himself, and must be exactly the same as it is in himself.  Otherwise God would have an inadequate idea of himself, which wouldn't make sense.

Here's how Frank Sheed puts it:

"An idea is, so far as we can make it so, the mental double or image of the object we are contemplating; it expresses as much of that object as we can manage to get into it.  Because of the limitation of our powers, the idea we form is never the perfect double or image, never totally expresses the object, in plain words is never totally adequate.  But if God does, as we know from Himself that He does, generate an idea of Himself, this idea must be totally adequate, in no way less than the Being of which it is the Idea, lacking nothing that that Being has.  The Idea must contain all the perfection of the Being of which it is the Idea.  There can be nothing in the Thinker that is not in His Thought of Himself, otherwise the Thinker would be thinking of Himself inadequately, which is impossible for the Infinite.  Thus the Idea, the Word that God generates, is Infinite, Eternal, living, a Person, equal in all things to Him who generates It - Someone as He is, conscious of Himself as he is, God as He is." (Frank Sheed, Theology and Sanity, pg 66, Sheed & Ward, 1973).

The next step is really important: the Father knows and loves, so his idea of himself knows and loves.  In other words, the idea is a person.  The thinker and the thought are distinct, but they are of the same nature.  The idea, or word, of God, is the Son.  God has always been thinking, so he has always been Fathering the eternal Son.

So the Second Person proceeds from the First by way of a word, or by way of knowledge.  The other primary operation of a pure spirit is love.

The Third Person
Between the two infinite persons of the Father and the Son, there is an infinite love.  Since in their mutual love they give everything they have, then the love between them is perfect, so that love produces an eternal person as well, the Holy Spirit.

Again, here's how Sheed puts it:

"The First Person knows Himself; His act of knowing Himself produces and Idea, a Word; and this Idea, this Word, is the Second Person.  The First Person and the Second combine in an act of love - love of one another, love of the glory of the Godhead which is their own; and just as the act of knowing produces an Idea within the Divine Nature, the act of loving produces a state of Lovingness within the Divine Nature.  Into this Lovingness, Father and Son pour all that They have and all that They are, with no diminution, nothing held back.  Thus this Lovingness within the Godhead is utterly equal to the Father and the Son, for They have poured Their all into it.  There is nothing They have which their Lovingness does not have.  Thus Their Lovingness too is Infinite, Eternal, Living, Someone, a Person, God.  Observe that here again we are still within the Divine Nature.  For love is wholly within the nature of the lover.  But this love wholly contains the Divine Nature, for God puts the whole of Himself into love."

Summary
So one way to think of the three persons of the Trinity is as Existence in the Person of the Father, as Knowledge in the person of the Son, as Lovingness in the Person of the Holy Spirit.  Or you can think of the nature of God as perfect Thinker, perfect Idea or Word, and perfect Love.  The inner life of the Trinity consists of the infinite interflow of knowing and loving among three persons who have the same nature and are one God.

That's enough for now, I think.  There's a whole lot to try and get your head around when you think about the Trinity.

Further Reading

• Frank Sheed, Theology For Beginners, Chapters 4, 5 and 6 - excellent introduction to the Trinity.
• Frank Sheed, Theology and Sanity, Chapters 3, 6, 7, 8 and 9 - more detailed and in-depth discussion.
• Glenn Miller - Christian Distinctives: The Trinity - Explores the Scriptural basis for the doctrine of the Trinity.
• Aurelius Augustine - De Trinitate [On the Trinity] - the classic exposition of the Trinity from one of the greatest Doctors of the Church.
• G.H. Joyce - The Blessed Trinity [1913 Catholic Encylopedia article] - An excellent, and (relatively) brief, summary of the doctrine and its history.
• The Catechism of the Catholic Church, paragraphs 232-267.