There was an old nursery rhyme I knew as a kid. “What are little boys made of? / Snips and snails / And puppy dogs tails / That’s what little boys are made of. / What are little girls made of? Sugar and spice / And everything nice / That’s what little girls are made of.” This 19th century rhyme tries to capture something of the essence of little boys and girls. Boys are dirty and rough. Girls and sweet and proper. There’s a lot of truth to this. But people are more complex than this. Boys can be dirty and rough. They can also be kind and thoughtful. We don’t tend to think of people as being whatever particular attribute they may possess. When we say boys are dirty, we mean dirty in an adjectival way. We do not mean that boys are dirt. When we say they are rough, we do not mean it in a substantive way. We mean they act in ways that can be described as rough. They do not comprise the sum total of all that is meant by “rough”-ness.
Augustine explains this idea using the metaphor of a cup and liquid. If you fill up a cup with tea, the cup and the tea remain distinct. A cup does not become what it contains. And the contents add something to cup that was not there before. There is division between the one and the other. The tea is inessential and optional to the cup. This is how we can think of attributes with ourselves. You could add milk or sugar to this tea. Or you could separate the composition into its various parts. But the cup never becomes its parts. We are composite and complex in this way. But this is not the way with God and his attributes.
God is simple. This is kind of an odd statement to make because explaining it can get very complicated and complex. God is simple. When we make this statement we are saying that God is not composite or composed of parts. Unlike the cup of tea, there are no components or division in God. So, in discussing his attributes we are not describing a part of God. You cannot describe God with a pie chart explaining that he is 15% holiness, 15% love, 10% wrath, etc. This would make God into different parts that comprise the whole of God. But God is simple. This means that God’s attributes are not inessential or optional. God is his attributes. It is not just that God has love. God is love. Not just that God is truthful. God is the truth. God holds these attributes with perfection, so every attribute is identical with God’s essence. The simplicity of God “is not merely a negative statement – God is without parts – but a positive one as well: God is identical with all that he is in and of himself. In the purest sense, God is one; he is singular perfection.”
Why is this important for our understanding of God? As we saw with the immutability of God, no one attribute of God is really understood in isolation from the others. Each overlaps and contributes to a fully faceted picture we have of God. God’s simplicity maintains his perfection, supremacy, and aseity. Aquinas points out that every composition requires a composer. He also notes that every composition is subsequent to its components and dependent upon them. This means that if God consisted of parts, those parts had to pre-date him in time; meaning they were not created by God or exist apart from God. And someone or something had to combine these parts to make God. Likely, you are grasping why the simplicity of God is crucial to an orthodox understanding of God. God cannot depend on parts.
Parts also make God divisible. If God is divisible, then God is destructible. Anything physical in our universe is composite. It is, therefore, transient, impermanent, reducible, mutable, and fragile. If something is divisible, then it can be taken apart. If it can be dismantled then its parts are subject to decomposition. Complexity results in degradation.
We can see this if we fail to apply simplicity to God’s goodness. The late medieval era saw scholars debate a question like this: Is something good because God wills it to be good? Or does God will something because it is good? This seems to put God between a rock and a hard place. If you say something is good because God wills it, then God appears arbitrary. If you say God wills something because it is good, then God is subservient to a standard of goodness that exists outside of God.
The simplicity of God helps to answer this paradox. “God neither obeys the moral order, nor does He invent it. He is Goodness Itself, and all else that is good is good in imitation of God’s nature.” God is not bound by some external concept of “goodness” that exists over him. Nor did God create “goodness” as something separate from him. God is goodness. Goodness is good because it mimics the nature of God. This could be repeated with every attribute of God. God is his perfections because he is simple. “Hear, O Israel: The LORD our God, the LORD is one” (Deut. 6:4).
 Bavinck, Reformed Dogmatics, 2.173.
 Barrett, None Greater, 76.
 Rogers, K., quoted in Barrett, 80.