At the heart of theology — which is the study of God — is the most proper object of study and understanding: God himself. Without it, theology literally becomes an impossibility. The doctrine of the Trinity, the most central aspect of our faith, while revealed in Christ and made known to us through Tradition and Scripture, was one of the most hard-fought doctrines in the early Church. The Christological heresies of the early Church were just as much heresies about who God is, as the Church wrestled with the issues: How can God be Father, Son, and Holy Spirit while also holding to the idea that God is one? Also considered is: How does God and humanity relate in the Person of Jesus Christ (this is something that becomes the field known as Christology, the study of Christ).
Talking about God in theology is an effort that requires great reverence on our part and a sober approach in our language. God is infinite, ever-greater than all of creation, and so we must recognize the limits of our language. So often, when discussing the question of God, we hear in popular understanding that God — or whatever people understand God to be — is so great a mystery that we can’t actually grasp it, that every religion is just a different way of approaching the great spiritual reality that the West calls “God.” Thus, as is often said, our language is insufficient to really say anything meaningful about God. So, according to this position, it doesn’t really matter what you believe about God, or what you say, just so long as you’re sincerely pursuing some spiritual reality.
This idea, as popular as it is, fails two tests: one of reason, the other of faith. The test of reason fails because it presumes that because God is so very other, that he cannot be known at all by human reason. Though it is true that God is greater than human reason can fully comprehend, it does not mean that man can’t know things about God: i.e., that he exists, that he creates, that we can know him through creation, etc. Furthermore, the idea fails the test of faith because it forgets the idea that though there is an ever-greater difference between God and his creation, there is also a similarity: the whole of creation bears the imprint of the creator, and that this creator desires to make himself known to the world by revelation. In the Catholic tradition, the response to this problem is known as doctrine of analogy. Just as we popularly understand that analogies have similarities and differences, so it is when it comes to us talking about God. We can say there is a real similarity between the words we use for each other and God. For example, we can say that however truthfully we understand the word “good,” it can be true of both us and God. But, because God is infinite and beyond comprehension, any similarity is always balanced out by an ever-greater difference. That is, the goodness of God is more different than it is similar to our understanding of goodness. This must always be held when talking about the Trinity: our limited categories can speak about God, but they can never fully grasp the reality either since God is ever-greater.
The heart of Trinitarian doctrine revolves around the relationship between the oneness of God and the threeness of Persons. For Christian theology, the Trinity is not three separate gods — a heresy known as tritheism — nor is it just one God who uses three different ways of appearing — a heresy known as Sebellianism or Modalism, after the idea that the Trinity has just three “modes” of appearing. Instead, the Church teaches that the Father, Son and Holy Spirit are each a real and distinct Persons, but each has the fullness of the one divine nature.
Trinitarian theology, along with the various aspects of Christology, help clarify the distinction between Person and nature so as to understand who God is and how we relate to him. Thus, in this distinction of nature and Person, the Church also reflected on how each Person relates to the others in the Trinity. Hence, in the Creed, we state that the Son is “eternally begotten,” that is, in all eternity he proceeds from the Father, while the Holy Spirit proceeds from the Father and the Son.
These reflections are important because they help us understand how God works redemption. This is how the Church has wrestled with the facts of the incarnation, death and resurrection of Jesus, and the sending of the Spirit at Pentecost. In all this we see a God who is a communion of love whereby each Person fulfills one particular task of love in the work of redemption, thereby making our redemption itself an act of love. And if one still thinks this to be abstract, then from the level of faith, we can see its effects on the human person when we remember that man is made in God’s image and likeness. A Trinitarian God, who is a communion of love, impacts and affects how we understand ourselves as human beings. To an extent, Trinitarian Theology affects everything and, therefore, requires care and humility when approaching it.
There are other questions that are important to this discipline which can’t adequately be dealt with in detail here, such as: How does God relate with creation? Does God change when Jesus takes on flesh (this is a question surrounding God’s immutability)? What do we mean that God is pure spirit, that he is eternal, the first cause? And so on. These are important questions that all have an impact on how we live our faith and are worthy of our study. By understanding them more deeply, though, they not only prepare us to give an answer to the world about who God is, but they aid us in worshipping God the way that we should.