“Simplicity” is the underlying definition of, or way to understand, the divine essence. Though hotly contested among modern and post-modern theologians (you can see the shift in the mid-19th cent. Bavinck even critiques Charles Hodge in a footnote about this very subject), simplicity was mostly universally accepted throughout Christendom. Recently Lewis Ayres has identified three organizational planks behind pro-Nicene theology, and simplicity is right at the top of the list. It is the statement that God is not composed of “parts,” nor do his attributes make up a composite. All of God is all of God, and each of His attributes is Him. “Simple” is thus opposed to complex or composite.
Simplicity is really another way to explain infinity. If God is outside of space and time, and thus always all that He is without bounds, then no “real” distinctions can be placed within His being. This means that while we can describe the way various divine attributes appear to us and how they interact with creation, we cannot, properly speaking, affirm that they are distinct, bounded off from one another, or “different” within God’s own self. Each of His attributes (love, power, will, justice) is identical with His essence, for they are divine. They are God.
Simplicity does not apply to the personal attributes, because those attributes are descriptions of the way in which the hypostases relate to one another. “Paternity” is the way to describe the Father’s mode of being. It is not an attribute of the divine essence. The same goes for ingenerate. That strange word was behind the Eunomian heresy, a form of Arianism. Eunomius said that since God the Father was ingenerate, that to be ingenerate was an essential attribute of deity. Anything generated, according to Eunomius, must not be God. The Cappadocians all combated this theory, but they did not merely deny simplicity, as some allege. Instead they maintained that “generation” was a personal attribute, unique to the Son. In fact, it isn’t even an “attribute” in the general sense, since it does not describe the essence but the personal mode of being of the one Son.
Gregory of Nyssa explains this in detail, but we’ll pick out a few expressions here. In fact, Nyssa relies on simplicity to explain how the Son can be co-equal with the Father:
For who does not know that, to be exact, simplicity in the case of the Holy Trinity admits of no degrees. In this case there is no mixture or conflux of qualities to think of; we comprehend a potency without parts and composition; how then, and on what grounds, could any one perceive there any differences of less and more. For he who marks differences there must perforce think of an incidence of certain qualities in the subject. He must in fact have perceived differences in largeness and smallness therein, to have introduced this conception of quantity into the question: or he must posit abundance or diminution in the matter of goodness, strength, wisdom, or of anything else that can with reverence be associated with God: and neither way will he escape the idea of composition. Nothing which posses wisdom or power or any other good, not as an external gift, but rooted in its nature, can suffer diminution in it…
Simplicity is what allows the Son to be “given” the Father’s nature without suffering any subordination. The nature which is given is indivisible and infinite, and thus for the Son to be given it is for the Son to be given all of it.
Augustine says this about simplicity:
But God is truly called in manifold ways, great, good, wise, blessed, true, and whatsoever other thing seems to be said of Him not unworthily: but His greatness is the same as His wisdom; for He is not great by bulk, but by power; and His goodness is the same as His wisdom and greatness, and His truth the same as all those things; and in Him it is not one thing to be blessed, and another to be great, or wise, or true, or good, or in a word to be Himself.
A more recent Reformed theologian, Herman Bavinck, stands in continuity with this tradition:
When we speak about creatures, we distinguish variously between what they are and what they have; e.g., a human being remains a human being even though he has lost the image of God and become a sinner. But when we speak about God, we must maintain that each of His attributes is identical with His being. God is all light, all mind, all wisdom, all logos, all spirit, etc. In God “essence is the same as wisdom, the same as goodness, the same as power. One and the same thing is said whether it be stated that God is eternal or that He is immortal or good or just.” Whatever God is He is completely and simultaneously. “God has no properties but merely is essence, God’s properties are really the same as His essence: they neither differ from His essence, nor do they differ materially from one another.”
As can be seen in Bavinck’s words, simplicity is unique to the divine. Everything else, existing within space and time as it does, admits of degrees and parts. Not so with God.
This is important, again, because of God’s spiritual and infinite nature. He does not exist within space and time, and thus it is illegitimate for us to place division points within Him. There is no “mid-point” for infinity. There is no “more” or “less,” nor can we actually distinguish where one attribute “stops” and another “begins.” This is a mystery to be sure, but it is not irrational because we are discussing something that is not within space.
Stephen Holmes has defended the doctrine of simplicity from modern philosophical critiques, noting that we are not referring to God as a simple monad in the earthly sense of that term. God’s attributes do not merely reduce down to one basic thing that stands “behind” them all. Rather all of the attributes are just the essence. Holmes uses the example of wine tasting:
Wine-tasting notes, for example, often contain a number of words and phrases, some of them apparently in tension if not simply contradictory, which are the attempt of the taster to express a single, but immensely rich, experience, viz. the taste of a fine wine.
The analogy is limited of course, but it is still true that once wine is fermented, fined, and filtered, you no longer have a collection of various ingredients but the one wine. It admits of many characteristics, but it is still the one thing.
The older analogy, and the one with the most Biblical support, is light. Light is one thing. It may appear in different places with different shades, but it is still the one thing. The most impressive display of multiplicity in the single light is the rainbow. Though a spectrum of colors appear, there are no bands, no marks of separation between the colors. The divine nature is most comparable to this because, as it appears to have many differences to our perception, they are all the same thing, God. There is no variation or shadow of turning.
Now this subject is, ironically, very difficult to explain. That’s the classical doctrine of simplicity though. The more simple something is, the harder it is to grasp. It is important for understanding how God is one, as well as how the persons of the Trinity can be unique yet still possess the fulness of the divine nature. As soon as simplicity is denied, one is forced to speak of parts of degrees of God, and thus one has already pulled the divine into created categories. Modern critiques consistently do this, remaking God into the image of creation, but the Christian position is the reverse. God is the unique and a se, while all of creation reflects Him.