The doctrine of the Trinity is a doctrine that provides us with a road map of how we can speak about God. It comes from scripture and its identification of the God in whom we believe is directly from the Apostolic witness found in the biblical narrative. In a world full of false deities that did not create the world nor suffer, die and rise and now sit in the judgment seat nor speak through the prophets and call, gather and fill the church, we look to this doctrine to know who our God is. It is a truth about God that we may hold on to as we read scripture so that we know to whom they testify. It is the reality of the God who creates, justifies, sanctifies, calls, forms and lives in us.Hear O Israel, Adonai is our God, Adonai alone! (Deut 6) When we talk about God, we begin here, with the God about whom the Shema is speaking. This is a proclamation that the God of Israel is this one, specific God. Adonai is God and it is Adonai and no one else. Adonai is not god of the river or the thunder or fertile crops. Adonai is not even King of the gods. He is God, period. God and God alone. Beginning here is significant because without this foundation in the ultimate uniqueness and singleness of the God of Israel, we would have no reason to speak of the trinity and would, instead, speak of two or three gods without any second thought. Yet, Jesus comes into time to a people of the One God, a people of the Decalogue, the first commandment of which is that there will be no other God but Adonai, and we proclaim that God the Father, God the Son and God the Holy Spirit are not all gods but are, indeed, God.We as Christians profess a belief in Jesus Christ as God. This is the fundamental characteristic of being Christian. In the second article of the Nicene Creed, we profess that this Jesus Christ is the only son of God. It seems that we should start with the first article of the creed, which begins with statements about God the Father. However, because we are only able to speak of that person of the trinity because of Jesus, it may in fact be better to begin at the second article. The creed tells us about a particular man with a particular history and gives us a synopsis of the narrative of his story in order that we may know the individual about whom we are concerned. He was the one whose mother was a virgin, who was put to death via crucifixion by a man named Pontius Pilate. He is the one who rose from the dead and ascended to heaven and now sits at the right hand of the Father as judge of all, both living and dead. Not just any man who happened to be very good or a symbol or an idea of a perfect man, but this particular Jesus of whom this narrative speaks.The next question is: why do we believe that this Jesus is God? This is a very complex question and the full answer to it goes far beyond the scope of this paper. However, it can be said that the biblical witness and the early church show us that Jesus is the one who was foretold by scripture, the Son of God to whom Adonai says, “you are my son: today I have begotten you” (Psalm 2:7). He is the one who has defeated death and is the first born of the resurrection. He is the one in whom all the promises made by the God of Israel are fulfilled (2 Cor 1:20). A community forms around this man Jesus that worships him and identifies him specifically as being one with this same God of Israel. This is found explicitly in scripture, one example of which is in Revelation when the one on the throne and the Lamb are being worshiped together (Rev 5). Jesus also speaks about himself as being one with God (John 10:30) and using the name given by God to Moses, I AM, as his own (John 8:58).Now we must ask: who is the Father? The creed tells us not only that he is God Almighty and that he created everything in existence, but it also tells us that he is the one who eternally begets the Son, Jesus. In using the phrase “begotten, not made, of one being with the Father,” the creed is illustrating not that the Father begot Jesus in sense that there was a point of origination as there is with humans who have offspring. Rather, the phrase speaks to the Father and Son being of the same essence. It is like human begetting in that humans beget humans and not kittens or coffee tables. The offspring of a human is another human. What God begets is God and it is all of the same essence, however there is not a point pre-begetting. The Father is also one half of all the “from” statements in the second article: God from God, Light from Light, True God from True God. All of these statements further affirm that the Father is of one being with the Son. The Father’s divinity can be most easily (but not solely) established by his status of being the creator of quite literally all things. This makes him an un-created and, therefore, divine being. We equate him with the God of Israel because Jesus, who was sent by the God of Israel, repeatedly calls him Father and equates the two. Jesus is sent by Adonai and begotten by the Father. They are one in the same.
The Third Article of the Creed confesses a belief in the Holy Spirit. We say that he is the Lord and the Giver of Life. If Jesus comes from the Father and the Father begets the Son, both eternally, the where does the Holy Spirit fit in and why is he God, too? We say that he proceeds, or goes forth from, both the Father and the Son. He is worshiped and glorified right along with God the Father and God the Son, illustrating his divinity. Scripture bears witness to the actions of the Spirit and, throughout, they are divine actions. He is one who sanctifies and creates yet needs no sanctification nor creation. He gives life and light to the world. He does things that are divine and to say that the Holy Spirit is not God would be to say that a created being can do divine things.
It appears that we now have three gods: a Father God, a Son God and a Spirit God. However, this is the God of Israel we are dealing with, the God who proclaims himself to be the one and only God. Adonai is one God, not three. So, how can we reconcile this? We could say that Father and Son and Spirit are the three ways in which God shows himself to us, but that would be modalism and this would deny the very relationships that define the Three. We could just say it is a paradox, but that would be giving up too soon. Instead, to reconcile what appears to be conflicting statements, we must talk about the ousia of God—what God is, and the hypostasis of God—who God is.
God is three persons, Father, Son and Holy Spirit, and yet is one God. When we use the word ousia, we are talking about the “what” of God. What God is. God is one and, therefore, if we are to say that the Father, Son and Holy Spirit are all three God simultaneously while still maintaining that this is Adonai and he alone is God, then we say that they are all of the same ousia or being. The Father is homousion, or of the same substance as, the Son. The Son is homousion with the Spirit who is also homousion with the Father. This means that whatever we say is true of one is true of all because they are all three of the same essence.
This word, “ousia”, does not by itself fully give an understanding of the trinity because it does not account for the distinctions present among the Father, Son and Holy Spirit. While there is one ousia, there are three hypostases. This is the “who” of God and marks the Three as distinctively separate. An example of how these two words “ousia” and “hypostasis” are used can be seen is by employing the analogy of categories and subcategories. If we say that there is a category of cats, we can describe what cats are. This would be the ways in which all cats are homousian with one another: they have whiskers, they purr, they meow, they are mammalian, they are furry, etc. Then, if I speak about the subcategory of my cat, Gracie, I am speaking of her as a distinct individual animal with a personality and specific narrative and, therefore, her own hypostasis. All the things one can say about the category of cats would be true of her, but there would be certain distinctions in the subcategory of Gracie based on her individuality. This analogy is helpful only so far. We cannot categorize God because he is not a part of the created world and there is nothing with which to compare him. However, we can see how the two words “ousia” and “hypostasis” work together with this analogy. This terminology allows us to speak of the Three as individuals, specifically by how they are related to one another. The Father is defined as begetting the Son, the Son is defined as begotten by the Father and the Spirit is defined as proceeding from both the Father and the Son.
There will always be something about the trinity that is beyond our grasp. C. S. Lewis wrote about the Flatlanders who lived in a two dimensional world and could only barely grasp the idea of a three dimensional cube. They could never visualize it because in their world of only two dimensions it was truly beyond their ability to understand. In the same way, the true reality of the One God who is Three is not something that we can visualize, make a model for or fully describe in a human formula and we must always qualify and redefine what we say in order to grope for the reality that is God.