The Issue
The idea of God as a Trinity has often led Jews and Muslims to accuse Christians of polytheism. Of course, the idea of three being one is a difficult and seemingly contradictory teaching. And this problem even persists with shared Biblical heritage supporting it. In the 26th verse of the Bible, God says “let Us make man in Our own image and likeness,” for example (Gen. 1:26). So, if the issue of the Trinity can cause a rift within Abrahamic religion, how much more of a stumbling block must it be to someone who is unsure of God in the first place?
The question is very simple: “how can three be one?” Often, the response is something like “it’s a mystery.” But this is a butchering of what “mystery” means in the context of religion. A mystery is not a cold case where we don’t have a clue what’s going on; a mystery is something we can perpetually think and learn more about, something utterly inexhaustible to our finite minds. The Trinity – God’s own nature – is, of course, one such infinite, inexhaustible mystery. That does not preclude our being able to think about it; rather, it invites us to think about it more. And think about it we have! Below is a the basic answer as to how three can be one.
Nature and Person
There are two things we must know about God before discussing the Trinity. I am presupposing you understand that God is a self-evidently existing being who moves reality through ideas. If that statement confuses you, please read this and then come back. Now, what I have just described is God’s nature. Nature is the “what-ness” of a thing. If I ask you what a human being is, I’m asking you to describe the nature of a human being – a rational, bipedal animal. However, when I ask you that question, my nature isn’t asking it. Rather, I – the person with the nature – am asking the question. Nature answers “what?”; person answers “who?”
Now here’s the confusing bit: God actually is identical to His nature, on account of absolute simplicity (again, see the link above if necessary). Yet, within God, there are three persons. There is one “what,” the “what” is the same as the “who,” and yet there are three “who’s.” How is this possible?
The Trinity
God is immaterial, and what is immaterial is mind. God, being absolutely simple and omniscient, knows Himself perfectly (again, see the link above if needed). This means that whatever is in God principally must be contained within His idea of Himself – otherwise, He would not know Himself perfectly, and would not be God. Therefore, in order to avoid concluding that God’s self-image is imperfect, we must posit that His self-image has all the attributes He has (note, of course, that God doesn’t actually have “attributes” but we nominally assign attributes in our concept of Him). This means that God’s self image is alive, absolutely simple, omniscient, perfect, loving, eternal, immutable, and so forth. Unlike our self-image, which is merely a thing, His self-image is a person, a who, a perfect reflection.
Next, recall that the will is the power which makes manifest the knowledge and judgment of the knower. Now a human may have a particular image in his mind that he wishes to paint, but by the impediment of a shaky hand, his will manifests his thought imperfectly. Or, a human may will something, but through a defect in his knowledge, fail to will it perfectly. God, on the other hand, is perfect, so there is nothing which could possibly impede Him from manifesting His perfect knowledge exactly as it is. And what does God know, and therefore love? Himself. So, God’s will, in loving Himself with nothing held back, perfectly manifests His own self, meaning His will is His own self – a third person.
So, God the principle, in thinking of Himself, generates Himself. And God the principle and God the generated love each other with one love, which is also identical to themselves (Summa Q28, A4). All three – thinker (Father), thought (Son), and love (Spirit) – are totally and wholly God; perfect, eternal, uncreated. There is no relationship of causality or creation here, such that one could be plucked out while leaving the other two intact. The foundation does not cause the walls and the roof, though they do rest upon it; likewise, the Father does not cause nor create the Son and the Spirit. The ONLY distinction between the persons is a distinction of relation to each other. And while these distinctions are real, they don’t break the unity of the essence (the “what”). These relations simply are the essence (Summa Q39, A1).
What is God? Father, Son, and Spirit. Who is God? Father, Son, and Spirit! This is how one “what” can be three “who’s.”
This leaves a question as to how God knows anything besides Himself if He is His only object of thought? How does God know who you and I are? Well, imagine you knew yourself perfectly. The fact of knowing yourself perfectly would involve knowing exactly how your friends, for example, would relate to you. Likewise, God, by knowing Himself perfectly, knows how all created forms relate to Him (Summa Q15, A2). Thus, He knows all things perfectly through a multiplicity of ideas contained within His one substantial object of thought: Himself.
Of course, we are not God, meaning there is a difference in how he knows us versus Himself. The Father, Son, and Spirit have a real relation to each other, whereas God does not have a real relation to creation, which is ordered to Him, not the other way around (Summa Q13, A7). And yet, when we are in a state of grace, this glorious Trinity – perfect, eternal, uncreated – condescends to dwells within our own souls. This infinite gap is not only mystically bridged by an unutterable miracle of grace, but bridged in such a way that God makes Himself our possession (Summa Q38, A1). What could this gift be compared to? What greater love could God exhibit than to stoop down to us, and place within us His own eternal life?