It’s 12:30 on a Sunday morning, and the organ has just teed up the closing hymn. Lunch is eminent; one song to go. With a low, hungry growl, you peel yourself up from the pew, and begin to sing:

‘Holy, Holy, Holy, Lord God Almighty,

Early in the morning my song shall rise to Thee.’

Morning?’ you think to yourself. ‘It’s nearly 12:30.’ You’re so distracted by your growling gut that you don’t register the faint whiff of confusion as you sing, in a small, hungry voice: “God in three persons, Blessed Trinity!” Suddenly, you blink confusedly. God in three persons? How can one God be three persons? But within a nanosecond, you’ve decided that 12:35 is too late to tackle a question that has vexed Christians for nearly two millenia. After all, there is a great deal about it that we just can’t say!

In this post, however, I will suggest that there is actually a great deal to be said about the doctrine of the Trinity, and not (at least, not only) by the experts. The doctrine of the Trinity, after all, isn’t some esoteric reason for theologians to pull their hair out. The doctrine of the Holy Trinity is about the very identity of the God who invites us to know and love him. When it comes to handling the doctrine of the Trinity rightly, then, the critical thing is the desire to know and love the Triune God. In this post, we’ll look at five critical questions that have helped Christians through the ages better know and love the Triune God. We’ll focus on the more ‘metaphysical’ questions since these are precisely the ones rarely articulated in the idiom of loving and knowing the Triune God. (Just don’t proceed on an empty stomach!)[1]

Here are the questions:

First, what is the difference between a substance and a person?

Second, how ought we to speak of the Holy Trinity?

Third, what exactly is a person?

Fourth, does the term person apply to God?

Fifth, what does it mean to know and love the Triune God?


  1. First, what is the difference between a substance and a person?

If we don’t look at this question carefully, we risk veering off into one of many well-worn ditches of ancient heresy. Take, for example, two such historic heresies: modalism and tritheism (though this is more of a family of heresies). A good illustration of modalism, or the view that God only appears as different persons, is the analogy to a teapot pouring water into three cups, or the man with three different roles: father, son, and husband. On the other hand, there is the error of tritheism, which views each divine person as a distinct god. Here, the best illustration is the famous ‘three leaf clover’ image of St. Patrick.[2] Both fail in the same respect: they fail to answer this question about substance and person properly. And the chances are, if we don’t look at the question carefully, we too will veer toward one of these positions (or another like them).


The concept of ‘person’ was one of the most important developments in early Christian thinking. Two words, one in Greek (hypostasis, ‘something that stands under’) and one in Latin (persona, which referred to an actor’s mask) both came to be synonyms for the concept of ‘person’. There are in the Trinity three hypostases or personas, three persons: the Father, the Son, and the Holy Spirit. Each term corresponds to another term that denotes God’s nature, one in Greek (ousia, or being) and one in Latin (natura, or nature). These were roughly parallel terms for the concept of nature or essence. These extrabiblical terms were never meant to substitute philosophical constructs for biblical witness. Rather, they were the Church’s effort to faithfully interpret who God had revealed himself to be, as the Father, the Son, and the Holy Spirit.

So, we have substance (ousia, natura) and person (hypostasis, persona)—but how are we to understand how three persons share one substance?

An important fourth century theologian, St. Gregory of Nyssa (c.335-c.395 A.D.), tackled this question. Sadly, Gregory has often been misunderstood as arguing that there are three independent beings in God, each with his own independent center of consciousness. This misreading of Gregory holds that the Father, the Son, and the Spirit share the divine nature like Peter, Paul, and James share the human one. But folks (as they say in my home state of North Carolina), that ain’t right. As Lewis Ayres has shown, this view (which has resurfaced in a popular and weirdly pervasive view called social trinitarianism) was quite foreign to the Council of Nicaea, where the question of how to define person and substance was front and center. In fact, Gregory stressed that there was only one mind, one will, one operation, that was shared by each member of the Trinity in an undivided way.[3] This is what we call the principle of inseparable operation. In a nutshell, this meant that the Father, the Son, and the Holy Spirit share one will, one operation—not three wills, or three operations. It is important that we see here that the doctrine of the Trinity does not teach three individuals, laboring cooperatively in all they do. That’s social trinitarianism, and quite frankly, tritheism. It is vital to underline that the Triune God is one God, not three.

2. In that case, how ought we to speak of the Holy Trinity?

As St. Augustine of Hippo (354-430 A.D.) recognized, even the most precise and elegant human speech will trip over itself when talking about the Trinity. The same goes for the most precise and elegant human philosophy. When speaking about the Trinity, Augustine pared down Aristotle’s ten categories of predication (substance, quality, quantity, relation, position, habit, place, time, action, and passion) to two categories only. Eight categories got the axe! The two remaining categories of predication – that is, of things we can say about stuff – were substance and relation. And even these needed to be refined of their usual, creaturely senses, so that we could say stuff about God rightly.

First, let’s take a look at substantial predication. Augustine thought that substantial predication was appropriate, at the very least, because of Exodus 3:14: “There is at least no doubt that God is substance, or perhaps a better word would be being; at any rate what the Greeks call ousia. . . . And who can more be than he who said to his servant, I am who I am, and, Tell the sons of Israel, He who is sent me to you (Exod. 3:14)?”[4] For Augustine, substantial predication wasn’t just saying that God exists; it was saying stuff about what sort of God exists: that he is good, just, merciful, loving, and so on. But these terms are not to be ‘predicated’ of each person in the plural, but of all the persons in the singular. ‘The Father is great, the Son is great, the Holy Spirit too is great; yet there are not three great ones but one great one.”

mako screen

Makoto Fujimura’s Trinity screen

That’s substantial predication, but let’s also look at relational predication. While substantial predication says stuff about what sort of God exists, relational predication says stuff about the relations that distinguish the persons from each other. Before, the focus was on the one substance; here, the focus is on the three persons. Substantial predication referred to someone with reference to himself; relational predication refers to someone with reference to someone else. So the Father, the Son, and the Holy Spirit are all ‘one great one’ (substantial predication). And, at the same time, the Father is really distinct from the Son and the Spirit; the Son is really distinct from the Father and the Spirit; and the Spirit is really distinct from the Father and the Son. Boethius (c.480-524), a student of Augustine’s, summed up this teaching this way: “the substance preserves the unity, the relation makes up the Trinity.”

3. So what exactly is a person?

This is the question on which the whole fabric of Christian salvation hangs. If we traced out what one potential wrong answer to this question might mean for our salvation, we might ask the question this way: Does Christian salvation mean rescue, or obliteration?

Boethius, who we met a moment ago, in a work entitled Against Eutychius and Nestorius, wrote that a person is “an individual substance of a rational nature.”[5] This definition, which was the definition that Thomas Aquinas had in mind when he got to the subject, comes at the end of a long argument against an ancient heretical group called monophysites. The monophysites held that Christ was formed from two natures, but also that he does not consist in those two natures. Rather, they said, Christ only had one nature; hence the name, ‘monophysite.’ The Catholics, in response, argued that both were true: Christ was formed from two natures, and he also consists in those two natures.

However, Boethius was dissatisfied with those who went to bat for the Catholics, so it was critical that he step up to the plate. Why? Because a monophysite Christ means a monophysite Christianity: that is, a Christianity that obliterates the human, just like a Savior whose divine nature obliterated his human one. This is A Clockwork Orange Christianity: the will is destroyed to solve the problem of being human. That is what was at stake for Boethius.[6] So in his argument against the monophysites, Boethius distinguished nature in four senses. The fourth and final definition is the one that concerns us.[7] Nature is the specific difference which makes a thing what it is. This is the definition both the monophysites and the Catholics were using. For example, one of the monophysites, Eutychius, argued that Christ did not consist in two natures because his divinity and humanity had united so much that they were now one new nature. In other words, the monophysites thought Christ’s nature was neither human nor divine, but a third thing altogether. This is Clockwork Orange Christianity: as the divine obliterates the human in Christ, so the divine can only obliterate the human in us.

clockwork orange alex

Boethius sees how problematic this is. So, with this definition of ‘nature’ in hand (i.e., the specific difference that makes a thing what it is), Boethius turns to define ‘person.’ ‘Person,’ Boethius insists, is predicated of a substance. And there are many kinds of substances: bodily ones (or not bodily), living ones (or not living). Some of those living substances are sensitive, and some of those sensitive, living substances are rational. Men, like God and the angels, are rational substances. But there’s one more distinction Boethius makes. All substances can be either universal or particular; the universal ones can be predicated of other things, while the particulars cannot. The fact that people are named shows that they are singular, unique, particular. Essences subsist in particular things; persons are the things in which essences subsist. And so, Boethius finally arrives at his definition of a person: “an individual substance of a rational nature.” (An individual substance, because it is unique and particular; a rational nature because it is a rational substance, as we have already seen, like God, angels, or men.) The only reason Boethius went into all this was to clarify the relation between substance (ousia, natura) and person (hypostasis, persona). This clarification was key if Boethius wanted to turn around and insist, as he did, that Christ is not only from two natures, but also consists in two natures. This affirmation is essential if we are to avoid a Christianity that obliterates rather than rescues.

4. Great – a person is “an individual substance of a rational nature.” But does this term apply to God?

Well, have you ever found yourself, in the middle of a prayer, beginning a sentence with “Heavenly Father,” and then ending it with, “Lord Jesus”? Praying to ‘God in three persons’ can be tricky business. Some surprisingly practical help comes out of the Augustinian tradition we’ve been discussing, especially through the medieval theologian, Thomas Aquinas (1225-1274 A.D.).

But let’s start, once again, with Boethius. In the fourth century, Latin theologians encountered a problem. They needed to translate the Greek word hypostasis into Latin, but its Latin equivalent was substantia. To use this word would have lost quite a lot in translation! So, Latin theologians had to find another word for hypostasis, and they duly chose subsistence. Boethius was the one who hammered out a series of correlations between Latin and Greek terms in Trinitarian theology that have stood the test of time. Many later scholastics – Albert the Great, Bonaventure, Thomas Aquinas – worked with Boethius’s correlations: so, “the Greek work ousia corresponds to the Latin term essentia (essence); hypostasis corresponds to subsistentia (subsistence); and prosopon corresponds to the Latin word persona (person).[8]” The second was particularly important for Aquinas when it was time to think through whether or not the term ‘person’ applies to God.

Boethius’s definition of person (‘an individual substance of a rational nature’) might lead us to believe that he thought of the Trinity as a collection of distinct centers of consciousness, who relate to each other merely as members of a commonly shared nature. As we’ve already seen, this view was foreign to Nicene thinking, yet persists to the present day in what we’ve called ‘social trinitarianism.’ But Aquinas saw through the risks of the definition, and used the term, provided it was applied to him in a divine and not a creaturely way. Aquinas, reaching back to Boethius, argued that person refers to an individual, to whatever it is that distinguishes this one from that one within a common nature. What distinguishes the three persons within the Trinity, Aquinas says, is their relations of origin, or personal properties. What distinguishes the Father is his paternity; the Son, his filiation; the Spirit, his procession. And since these relations are the things that distinguish the Father, the Son, and the Spirit from one another, Aquinas defines the divine persons as ‘subsisting relations.

hipster dad

The upshot of all this is that the word ‘person’ really must be used of God in a divine and not a creaturely way. For example, I am a father of two children. But once, I was not a father. Paternity, therefore, is an add-on to my human person, however important it is to my identity now. But God’s paternity is not an add-on; the relation of paternity (or filiation, or procession) subsists in God. So, while the Father, the Son, and the Holy Spirit are really distinct on the basis of these relations, each is also identical with the essence of God. It’s not that there’s a fourth thing, some ‘essence’ floating around that they partake in.[9] Rather, the one God has no existence apart from his threefold subsistence as the Father, the Son, and the Holy Spirit. So, to put some wheels on Aquinas’s thinking about subsisting relations, we turn to our fifth and final question.

5. What does it mean to know and love this God?

The late John Webster remarked that God’s acts introduce us to his presence. In other words, we meet the Triune God – Creator, Lord, and Savior – through his acts. One such act, Webster said, was that great act reflected upon by the colonial theologian Jonathan Edwards, and drawn from the riches of the Protestant scholastic tradition before him—”the eternal covenant of redemption in which the Father and Son design to execute saving work in relation to God’s creatures.”[10] This covenant worked out in the larger economy of God’s glory. “God is the infinite ocean,” Webster said, “into which his saving work empties itself.”

When we reflect on God’s creating, saving, and restoring work, we do not tidily divide these up, respectively, amongst the Father, the Son, and the Holy Spirit. Rather, we say, with Augustine, that the external works of the Trinity are undivided (opera Trinitatis ad extra indivisa sunt). Instead of neatly tying certain actions to the divine persons, Augustine’s insight helps us to see how, as our Creator and Rescuer, God’s tripersonal being shines through his work: as the Father sends the Son to redeem his Church; and as the Father, together with the Son, sends the Spirit to sanctify her.

Baptism of Jesus

Even more wonderfully, there is a double dynamic to the Holy Trinity. In the divine missions, we see the relations of origin that constitute the Godhead: the Father eternally generates the Son, and the Father, with the Son, eternally breathes the Holy Spirit. This is the descending dynamic, but there is an ascending dynamic, too:

“The relations of origins also constitute the “whither” of the divine missions insofar as they provide the divine exemplars and goals of those missions: the goal of the Son’s redemptive mission is to make us sons and daughters in order that he might be the firstborn among many brothers and sisters (Rom. 8:29; Gal. 4:5); the goal of the Spirit’s sanctifying mission is to embrace us within the fellowship of the Father and the Son, pouring out the Father’s love into our hearts (Rom. 5:5), and awakening within us the Son’s filial cry of “Abba! Father!” (Gal.4:6).”[11]

These five questions are helpful to ask, because they lead us to respond to God in worship. What is critical in the eyes of the Triune God, after all, is not an appetite for speculation about Him, but a deeper knowledge and love of him.

[1]I have relied on Scott Swain’s wonderful chapter, ‘Divine Trinity,’ in Christian Dogmatics: Reformed Theology for the Church Catholic (Baker Academic, 2016).

[2] See Tim Challies’ Visual Theology chart on the Trinity. If you’re itching to waste time, check out Lutheran Satire’s video mocking St. Patrick’s bad analogy.

[3]Gregory of Nyssa, Letter 38.

[4]Augustine, De Trinitate 5.1.3, quoted in Swain, Christian Dogmatics, p.95.

[5]Boethius, Contra Eutychen et Nestorium, 3.4-5.

[6] On this point, see C. Fitzsimons Allison, The Cruelty of Heresy: An Affirmation of Christian Orthodoxy (Harrisburg, PA: Morehouse Publishing, 1994), pp.106-8.

[7]Ralph McInerny, Boethius and Aquinas (Washington, D.C.: Catholic University of America Press, 2012), pp.99-100.

[8]Gilles Emery, The Trinitarian Theology of St. Thomas Aquinas, trans. Francesca Aran Murphy (New York: Oxford University Press, 2007), pp.112-3.

[9]The Fourth Lateran Council (1215) explicitly ruled out the idea of a ‘quaternity,’ or three divine persons plus a divine essence.

[10]See Webster’s Kantzer lectures at Trinity Evangelical Divinity School.

[11]Swain, Christian Dogmatics, p.104.

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Sam studies post-Reformation Protestant theology at Cambridge University.

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