There is perhaps no century exceeding the importance of the 4th century with respect to the expression, defense, and solidification of trinitarian theology. Though there have always been faithful defenders of the doctrine of the Trinity in its classical frame, discussion of the nature of the triune Godhead has captured the interest of a larger portion of modern commentators. One doctrine dear to the 4th century church fathers, although not as accepted among its modern commentators, was the doctrine of God’s simplicity.
Divine simplicity claims that the divine nature is not made of parts like all created natures, and that there is no division in the divinity. At its core, divine simplicity is concerned with asserting that God is utterly different than we are in that he is what he is in fullness. We are composite, or made of parts, and thus changeable. We are body and soul, and we receive holiness, grow in maturity, learn goodness. We are measureable, confined to be where we are. Different parts of us grow and diminish throughout time. None of these things are true of God. He is only uncreated being, and holiness and goodness have always found their source in him. God is what he is; he is his attributes; love, holiness, goodness, eternality, knowledge, being, love, life. Therefore, there is no change in God, and no possibility of his not being who he is. He is pure being, completely and only God.
While metaphysical, philosophical categories provide the language of simplicity, the theologians treated in this paper saw defending God’s simplicity as a biblical necessity to maintain the distinction between Creator and creature. It is specifically separation between creature and Creator that allows for God to be triune, whereas created beings could never be so. It should be stated that each of these men defended the view that God is one simple and divine substance, and that this substance exists as three persons, each being fully God and distinguishable from each other only by their relations to one another. This belief was at the center of the controversies of the 4th century. This short essay will survey some of the ways that Basil of Caesarea, his friend Gregory of Nazianzus, and Augustine of Hippo utilized divine simplicity to defend the doctrine of God’s incomprehensible, majestic triune nature.
Basil the Great
One accusation leveled at trinitarian theologians was that of tri-theism — the belief that the Father, the Son, and the Holy Spirit are each their own, separate god. Basil defends against this by arguing that God is one “not in number but in nature.” Quantity as a category is related to bodily nature, in that it is only when a thing is contained, or “circumscribed,” by created boundaries that it can be counted as an instantiation of a certain nature. For instance, men share the same nature, but are able to be counted—“One, two, three…” by means of the boundaries between where they are and where they are not. They have been created, and as created are composite, and can be numbered separately. A simple nature, on the other hand, is unable to be measured in the same way, as only what is created is composite and has a material and circumscribed nature.
Similarly, Basil argues, referring to the Father and the Son, that it is ridiculous to suppose that it is “not in line with piety to compare the one without form to the one without form… [or] the one without composition to the one without composition.” A nature without form, size, or composition has nothing to set it apart from another nature that is also without form, size, or composition. One woman is told apart from another woman by the fact she is sitting in one place, and the other is sitting in another; one is taller and one is shorter; one is older or wiser or smarter than the other. But there are no ways to measure those differences between two simple substances. So if the Father is a simple substance, and the Son is a simple substance, Basil must find another way to compare them. He suggests that there are no grounds for comparison apart from “identity of power.” In this case, “Christ is the power of God.” Since the power in Christ is the power of the Father, and the “divine is free from quality” by which to judge the natures similar or dissimilar, the natures are therefore identical, consubstantial. If we “confess identity of nature and … [reject] the composition of the Father, God in substance, Who begat the Son,” we also must confess that the Son is likewise “God in substance. From this consubstantiality is proved.”
Gregory’s doctrine of divine simplicity follows largely in Basil’s footsteps. One major use of divine simplicity in both Basil and Gregory is the role of circumscription and composition in determining divinity. Only created natures can be composed of parts (composite) and restricted by boundaries (circumscripted). But why this emphasis on non-compositeness? If God is made of parts,
[how is he not] therefore subject to be resolved into them again, or even altogether dissolved? For every compound is a starting point of strife, and strife of separation, and separation of dissolution. But dissolution is altogether foreign to God and to the First Nature. Therefore there can be no separation, that there may be no dissolution, and no strife that there may be no separation, and no composition that there may be no strife.
This explains a foundational reason why the church fathers defended the notion that the incomprehensible essence of God must be simple. If God is made of parts (compounded), he is subject to dissolution. God is utterly different from us; dissolution is as foreign to his nature as it is natural to ours.
Gregory also furthers Basil’s discussion comparing multiple, simple substances. Although many opponents were employing divine simplicity as a proof that God’s substance could only have one hypostasis (person), and that the Son, though simple, was a different substance, Gregory claimed that this was an impossibility. Since simple essences have no parts, “it is not correct to say of them that they are Like in particular and Unlike in another; but they are a complete resemblance, and should rather be called Identical than Like.” Two simple essences must either be completely unlike or “Identical.” You cannot have two simple, divine, immaterial, uncircumscribed substances, one begotten from the other, without being identical. Any nature that is not creature, therefore, must be creator, and if creator, then simple, and if simple, identical.
Augustine appeals to the concept of simplicity to argue that there can only be one uncreated substance. While there are two general kinds of substance, one created, and the other the Creator of the first, there can be many created substances and only one uncreated substance. Created substances are set apart by accidental qualities, which they receive from their Maker, while the immaterial and simple substance has no accidents to differentiate it from another immaterial, simple substance. The uncreated substance must be, by definition, immaterial, because there was no matter before it was created, and simple, because there is nothing above it to create it from parts or attribute different things to it. So, if there were two immaterial and simple substances, they would have to be identical, since there is no division in substance apart from the Creator-creature divide or the divisions of accidents in creation.
The language of accidents is also used to explain how God can be both a Trinity and be simple in substance. The Father or the Son cannot be called Father or Son “according to accident,” which would mean that they had at one time begun to be those things, introducing change and parts into the simple nature of God. Rather than fatherhood and sonship being accidental to the persons, they are simply designators of the relations in which they stand to one another. “Wherefore, although to be the Father and to be the Son is different, yet their substance is not different; because they are so called, not according to substance, but according to relation, which relation however, is not accident, because it is not changeable.” For Augustine, then, as with Basil and Gregory, it is the simplicity of the divine substance that allows for the one divine, simple substance to exists in three united but distinct persons.
Since the substance of God is simple, the attributes equal one another. “For to Him it is not one thing to be, and another to live … [or] to understand,” or to be blessed, good, powerful, eternal, invisible, etc. God is the “good which is simple, and therefore unchangeable,” because he is the standard by which all things are judged to be good. If his goodness was a part of him, then it would have come from somewhere, as parts assume being put together or taken apart. He “has not anything which [He] can lose, and because [He] is not one thing and [His] contents another.” But, as it is, God is simple—not made, but the Maker. What God creates is not simple, but what God begets is simple. It is in this way, the Son and the Spirit are both “another” to the Father but are not other “things,” because they are “equally with Him the simple Good,” begotten or proceeding, “unchangeable and co-eternal.” God is simple because he is what he has, with the exception of the relations; “each is what He has; thus, He is in Himself living, for He has life, and is Himself the Life which He has.”
The doctrine of divine simplicity was utilized extensively by these three churchmen as one way to defend the Trinity from attacks that utilized metaphysics and language wrongly to twist the clear witness of the Bible. It is precisely because God is not like us, simple rather than composite, that He can be one substance and three persons, without differentiation of substance between the three. Accidental categories only pertain to those who are created, limited, circumscribed, and able to be differentiated as specific instantiations of a particular substance. As soon as we attribute accidental properties to God, he is not only not Triune, but he is a creature, sitting below whoever attributed those qualities to him. Simplicity describes that God, as the first principle from which all else came, is pure being and pure act. He is immeasurable since he created all things used for measuring, and therefore exists as a Trinity in a way that is not accessible from below.
 Quantity is one of Aristotle’s ten categories of how matter, or being, can be known; simple substances cannot be counted in the same way. Basil is arguing that innumerability does not preclude, but in fact allows for, triune simple being. Non-simple substances could never be triune in this way.
 Basil, “Letter 8,” in St. Basil: Letters and Select Works, ed. Philip Schaff and Henry Wace, trans. Blomfield Jackson, vol. 8, A Select Library of the Nicene and Post-Nicene Fathers of the Christian Church Second Series (New York: Christian Literature Company, 1895), 116.
 Basil of Caesarea, Against Eunomius, 122:124. Cf. 1 Corinthians 1:24.
 Basil, “Letter 8,” 8:116.
 Ibid., 7:290.
 Ibid., 290–91.
 Ibid., 317.
 Ibid., 319.
 Ibid., 3:21.
 Ibid., 3:89.
 Augustine of Hippo, “The City of God,” 2:148.
 Ibid., 2:211.
 Ibid., 2:210.
 Ibid., 2:211.