I love a good riddle. There’s something so enjoyable about pondering over a riddle while anticipating the relief of the answer – digging more and more as the pieces slowly come together. Finally, something inside “clicks”, and the answer suddenly becomes all too obvious. The satisfaction is amazing.
But not every riddle and puzzle has an answer that simply “clicks”. Some puzzles go beyond our ability to reason. It’s not that they don’t make sense, or even that they aren’t logical. Rather, they exceed our ability to think. There are people who are intellectual geniuses, whose ability to comprehend things exceeds our own (Einstein, Mozart, Plato, etc). But there are mysteries that even geniuses can’t comprehend.
As a Christian, I see many objections raised against my faith. There are many mysteries, puzzles, and seeming contradictions arise in Christianity. At the very top of these mysteries is the belief that God is a Trinity (Father, Son, and Holy Spirit). How can a single God be three persons? This begs another question: If Christians believe God is a Trinity, why do they call themselves monotheists?
After all, Christians say that the Father is God, the Son is God, and the Holy Spirit is God. Sounds an awful lot like three Gods, doesn’t it? And if a polytheist is someone who believes in multiple gods, then why aren’t Christians polytheists?
To answer this question, we will need to do a little ground work to make sure we understand how Christians understand God as a Trinity. To begin, let’s look at the origin of this belief: we start with Jesus’ own words.
The Trinity is Revealed
Jesus did a lot in his earthly ministry. To name everything would be like trying to count the sand on the beach. But there is one thing in particular that he did that concerns our question: Jesus revealed who God is. Sure, God revealed himself to Israel through the Old Testament, but Jesus came to complete what was being revealed in the Old Testament.
There are many references to God’s identity throughout the New Testament. For instance, we see this in the Lord’s Prayer, as Jesus instructs us to address God as “Our Father in heaven…” (Matt 6:9). We also see Jesus revealed as God in Thomas’ famous exclamation to the risen Christ: “My Lord and my God!” (John 20:28). Similarly, we see the “Spirit of God” (the Holy Spirit) descend upon Jesus shortly after his baptism (Matt 3:16).
Just before his ascension, Jesus put a bow on the topic when he told the Apostles to “make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit” (Matt 28:19, emphasis added). Notice that he didn’t say “in the names…”. It’s a singular name, with three persons.
Jesus told us these things so that we could know God as he is: a Trinity. But at this point we should put up some caution tape. When I say Jesus “revealed” God, I mean that we couldn’t have figured this out on our own. We can use our reason to know that God exists, but we would never be able to figure out that God is a Trinity without Jesus having revealed it. We accept the doctrine of the Trinity by faith in Jesus’ words (for further discussion about faith, click here).
Some might be questioning the math here. Jesus revealed three persons, yet Christians say that there is only one God. What gives?
That’s a fantastic question. But before we can answer it, we need to do a little groundwork. We need to look at the difference between a “person” and a “nature”.
Person vs. Nature
I’ve been told that I’m freakishly similar to my father (a fact that I, too, agree with). We have the same build, tastes, and we both think that engineering is cool. But we have something even more fundamental in common: we share the same nature.
This can be said of all humans. We’re human because we all share our human nature. That’s what separates us from other creatures, like horses and cats (though some might disagree with me when it comes to cats…).
So what is a nature? Essentially, a nature defines what we are and what we can do. My human nature contains the fundamental characteristics that make me human.
What is a person? If a nature is what we are, then a person is who we are. A person is an individual, a “self”, distinct from others. Frank Sheed best sums up the distinction between person and nature:
Nature answers the question what we are; person answers the question who we are. Every being has a nature; of every being we may properly ask, What is it? But not every being is a person: only rational beings are persons. We could not properly ask of a stone or a potato or an oyster, Who is it? 
With that distinction, we can begin to make some sense of our math dilemma. There are three Persons in the Trinity: The Father, Son, and Holy Spirit. But there is only one Divine Nature that each Divine Person possesses. The three Persons are completely distinct from one another, but they are completely (and mysteriously) unified, possessing the one Divine Nature.
If you find this confusing, don’t worry. There is more to the story. We need to understand (with our limited human abilities) how the three Persons of the Trinity are related.
How it Works
The Father is the Origin
God the Father is the first Person of the Trinity, and our starting point. He is all knowing and all powerful. He completely possesses the Divine Nature and is rightly called God.
But his name, Father, also tells us something important about him: He is the source, the origin of everything, including his Son, Jesus.
The Son Proceeds from the Father
“Now wait a minute,” some might argue. “I thought you said Jesus was God. Now you are saying God the Father created Jesus?”
The short answer is no, God didn’t create Jesus. But Jesus proceeds forth from the Father. Allow me to explain.
This is where things get technical (bear with me). God is all knowing. To know everything infinitely, that means he must know himself perfectly and infinitely. So his own knowledge, or thought of himself would also be infinite in nature. And his idea of himself would be divine in nature, since he is divine. And his self-knowledge would also have to exist with him for all eternity, since God can’t, even for a moment, stop being all knowing.
Let’s piece together what we have so far. God the Father has an infinite, eternal, and divine knowledge of himself. God’s self-knowledge is so infinite and real that it (he) becomes a person. At this point, the beginning to St. John’s Gospel might make more sense.
“In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God” (John 1:1-2).
We see that the Word (Jesus) was with God for all eternity, and was God. Some might object that “Jesus is referred to as God’s word, not his ‘thought’ or ‘idea’.”
This is a good point. But we need to rethink our understanding of what “word” means. A word expresses a thought, but it does so by creating a sound; it requires a medium (usually air) to carry its sound waves. But Jesus was with the Father before anything was created (including air). So by calling Jesus God’s Word, we can rightly think of him as God’s idea or thought. Frank Sheed echoed this when he said:
So God utters a word – not framed by a mouth, of course, for God has no mouth. He is pure spirit. So it is a word in the mind of God, not sounding outwardly as our words sound, akin rather to a thought or an idea. 
We need to be careful here. I’m not calling Jesus a figment of God’s imagination (that would be heresy). Jesus is more real than the screen you are looking at. You see, God’s image of himself is infinite. It’s so much greater than anything we can comprehend. It’s so real that it (he) takes on personhood. He becomes a person. He is not the Father, but he is real, eternal, infinite, and divine. He is the second person of the Holy Trinity.
The Father and the Son exist together in heaven for all eternity, both completely possessing the Divine Nature. The Son proceeds from the Father through the Father’s intellect. They are a family together.
The Spirit Proceeds from the Father and the Son
The third person of the Trinity emerges when we consider that God is all powerful. His ability to do all things with infinite power includes his ability to love. This is where the Holy Spirit comes in.
Like any good family, the Father and the Son love each other, as a father loves his son, and a son loves his father. The Father infinitely pours his love onto the Son. The Son, in response, returns that love back to the Father. This mutual love is continual and eternal. We saw how immeasurable this love is when, in perfect, loving obedience, the Son gave up his own life on the cross.
This infinite exchange of love between the Father and Son is so great, that it takes on an infinite, divine nature. It’s a nature that is so powerful and real that it becomes a person. This is the Third Person of the Trinity: The Holy Spirit.
The Holy Spirit is the very love that flows between the Father and the Son. The Holy Spirit is not the Father or the Son. He is distinct from them both, yet he (as the Son is to the Father) proceeds forth from them. Frank Sheed sums this up well as he wrote:
As the one great operation of the spirit, knowing, produces the second person [Jesus], so the other, loving, produces the third [Holy Spirit]. But be careful upon this – the second proceeds from, is produced by, the first alone; but the third, the Holy Spirit, proceeds from Father and Son, as they combine to express their love. 
Don’t feel intimidated if you are having trouble wrapping your head around this – it’s deep stuff. The inner workings of the Holy Trinity make rocket science look like second grade math homework. God is a mystery, and we will never fully grasp the whole of his being, for we are finite beings trying to understand the infinite.
In short, Christians believe in one God, though he is three persons: three persons who share completely in one Divine Nature. Each person is completely distinct from the others, yet inseparable. The Father can’t simply cease to be the origin of the Son any more than he can cease to be infinite. And the Father and the Son can’t cease to infinitely love one another.
The Catholic Church echoes this teaching in the Catechism of the Catholic Church when it says:
The Trinity is One. We do not confess three Gods, but one God in three persons, the “consubstantial Trinity”. The divine persons do not share the one divinity among themselves but each of them is God whole and entire: “The Father is that which the Son is, the Son that which the Father is, the Father and the Son that which the Holy Spirit is, i.e. by nature one God. 
Each Person is entirely eternal and divine. Each Person is God. This sheds some light on Jesus’ words when he said to baptize that nations in the name of the Father and of the Son and of the Holy Spirit” (Matt 28:19). God is one. Yet God is made of three Divine Persons.
So Christians aren’t polytheists. We believe in One God. He is mysterious to us, yes, but shouldn’t we expect that of the infinite God of the universe? When Jesus revealed the mystery of the Trinity, he revealed something that we can only begin to grasp; he left us a great puzzle.
 “Theology and Sanity.” Frank Sheed: Author’s Page at Ignatius Insight. N.p., n.d. Web. 17 Apr. 2017.
 Sheed, F. J. Theology for Beginners. Ann Arbor, MI: Servant, 1981. Print.
 Catechism of the Catholic Church, paragraph 253