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! What follows is a basic explanation of how this aspect of divine revelation does not stand in opposition to reason.
Nature and Personhood
There are two things we must know about God before discussing the Trinity. I am presupposing you understand that God is an absolutely simple, 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. Yet, within God, there are three persons. We know this much not by reason, but by divine revelation, particularly as expressed by the Council of Nicaea. Yet, divine revelation cannot contradict reason, even if reason is impotent to confirm it without help. But this dogma seems, at least superficially, to contradict reason! How can it be said that God is one “what,” and that the “what” is the same as the “who,” and that there are yet three “who’s?”
Relation
We know already that we cannot predicate three essences of God, for there would thus be three Gods, and this is incoherent. There cannot be three foundations to reality, three sources of all being, nor three absolute truths. In fact, we cannot univocally predicate any accidents of God at all, since “whatever has an accidental existence in creatures, when considered as transferred to God, has a substantial existence; for there is no accident in God; since all in Him is His essence” (Summa Q28, A2). That is, we cannot say one Trinitarian person is wise and another powerful, because wisdom and power in reality describe the one simple essence, and are only distinct in our imaginations, used as metaphors to enliven our concept of God.
Now, there is one accident which does not modify its object absolutely: relation. A thing is relative when it is explained in reference to another thing – that is, when the relationship is not merely an idea, like the distance between two objects, but something intrinsic to the object which describes its order to another. We often clarify this by using the term “real relations.” Aristotle says real relations are either expressed by quantity – as the definition of six is “the number greater than five” – or act – as as the definition of father is “he who has begotten progeny.” Again, these relations are real – if there were no progeny, the father would not be father. Fatherhood is the act of being ordered to progeny. But this act does not modify the father’s substance absolutely, such as making him smaller or larger would.
This seems to give us our answer. Real relations do not modify a substance absolutely, and so can be in God. They could not be established by quantity, for God is immaterial, but could be established by an eternal act, for God is Actus Purus. Such an act could not extend outside God, lest God be outside God, but it could remain immanent. As I said before, whatever is an accident in creatures is essential in God. So we shouldn’t understand relations in God as accidental, but fundamental. Whatever real relations exist in God are not additive to His essence; rather, they are His essence. In fact, to avoid modifying the essence, we would need to say each relation contains all absolute divine attributes. That is, each relation is simple, omnipotent, wise – alive. Each eternal, immanent relation would have to be… a who!
Generation
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 to conceptualize His simple essence). 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 self-reflection.
So God the unbegotten – the Father – immanently generates God the begotten – the Word, or, the Son. In the natural order, a father more or less perfectly begets an image of himself. The Father we speak of here is God, who, as already discussed, produces a perfect image of Himself. That is, the image bears the semblance of the Father in all possible ways. There is, of course, only one aspect of Himself which the Father cannot supply: paternity. It is impossible to beget fatherhood; this necessary opposition between begetter and begotten is what distinguishes Father from Son. But just as their persons are distinct by what is exclusive to each, the third person is distinct by something Father and Son share: the power of will.
Spiration
The will (also called love) is the power which makes manifest the knowledge and judgment of the knower. Now a human may have a particular image in his mind which 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 will/love? Himself – Father to Son, and Son to Father. 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, the Holy Spirit.
How is this third person concretely distinguished from the other two? If the Father simply generated the Holy Spirit, then there would be no relation between the Son and the Spirit. Both would be “God the generated” and simply be two Sons. The verb used to describe the procession of the Spirit is “spiration,” and this is distinguished from generation by the fact that the Father generates the Son, but the Father and the Son spirate the Spirit. This is the perfect, indivisible, utterly immanent, eternal subsistent relationship that is God. The Father unites the Son and Spirit as their principle; the Son unites the Father and Spirit by mediating spiration; the Spirit unites the Father and Son in their shared spiration of Him.
So, God the Father, in thinking of Himself, generates Himself. And God the Father and God the Son love each other with a 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. This is because each person is their relation to the other – for example, if the Son ceased to be Son, the Father would cease to be Father, for the relation to a son is that very thing which makes one a father. We may think of the Father as a floor which supports two walls, but in no way causes them.
To be abundantly clear: all things predicated absolutely of God – eternity, simplicity, goodness, wisdom, power, and so on – are fully present in each person of the Trinity. God is one essence; there are not multiple Gods. The ONLY distinction between the persons is a distinction of immanent relation to each other. And while these distinctions are real, they don’t break the unity of the essence (the “what”). Rather, these relations are the essence (Summa Q39, A1). Because they are the essence, each relation has all the attributes of the essence, including life. So, what is God’s nature? Father, Son, and Spirit. And who is God? Father, Son, and Spirit! This is how one “what” can be three “who’s.”
Knowledge of Creation
This leaves a question as to how God knows anything besides Himself? 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. But to know this, you would have to perfectly know your friends. Likewise, God, by knowing Himself, knows all created forms by likeness to Himself (Summa Q15, A2).
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, for He would immanently remain exactly the same if it did not exist (Summa Q13, A7). And yet, when we are in a state of grace, this glorious Trinity – perfect, eternal, uncreated – condescends to dwell within us. 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 we compare this gift to? What greater love could God exhibit than to condescend to us, pour His own life into us, and know us as friends rather than mere creatures (John 15:15)?