Reformed theology, as properly expressed, considers the doctrine of God’s unity not as the classical formulation used by Aquinas and the Scholastics, but as a unity of being; in which all attributes of God are distinct in their display, necessarily interrelated but not identical to each other, despite being differentiated expressions of God’s singular, essential nature. The Scholastics (following the lead of earlier writers) may be summed up as follows: “It is commonly said in theology that God’s attributes are God himself, as he has revealed himself to us… It was further asserted by the Scholastics that the whole essence of God is identical with each one of the attributes, so that God’s knowing is God, God’s willing is God, and so on. Some of them even went so far as to say that each attribute is identical with every other attribute, and that there are no logical distinctions in God.” (L. Berkhof, Systematic Theology, 45)
This conception, however, is not that of the consistently Reformed position. I will give you Berkhof, Bavinck, and the inestimable Dr. Gill.
“Naturally, we should guard against separating the divine essence and the divine attributes or perfections, and also against a false conception of the relation in which they stand to each other. The attributes are real determinations of the Divine Being or, in other words, qualities that inhere in the Being of God. Shedd speaks of them as “an analytical and’ closer description of the essence.”
In a sense they are identical, so that it can be said that God’s perfections are God Himself as He has revealed Himself to us. It is possible to go even farther and say with Shedd, “The whole essence is in each attribute, and the attribute in the essence.” And because of the close relation in which the two stand to each other, it can be said that knowledge of the attributes carries with it knowledge of the Divine Essence. It would be a mistake to conceive of the essence of God as existing by itself and prior to the attributes, and of the attributes as additive and accidental characteristics of the Divine Being. They are essential qualities of God, which inhere in His very Being and are co-existent with it. These qualities cannot be altered without altering the essential Being of God. And since they are essential qualities, each one of them reveals to us some aspect of the Being of God.” – Berkhof, Systematic Theology, 45-46
Simplicity is not in conflict with the doctrine of the Trinity, for the term “simple” is not used as an antonym of “twofold” or “threefold,” but of “compound.” Hence, this simplicity does not conflict with the doctrine of the Trinity, for the divine being is not composed of three persons, neither is each person composed of God’s being plus the personal property; but the one and only uncompounded (simple) being exists in three persons; every person or personal property is distinct from God’s being not “in the object” but “in reason”; every personal property is indeed a “real relation,” but does not add “something real” to the “essence.” The personal properties “do not compose but only distinguish.” – Herman Bavinck, The Doctrine of God, trans. William Hendricksen, (Grand Rapids: Eerdmans, 1951), 172.
“God being a Spirit, we learn that he is a simple and uncompounded Being, and does not consist of parts, as a body does; his spirituality involves his simplicity: some indeed consider this as an attribute of God; and his spirituality also: and, indeed, every attribute of God, is God himself, is his nature, and are only so many ways of considering it, or are so many displays of it. However, it is certain God is not composed of parts, in any sense; not in a physical sense, of essential parts, as matter and form, of which bodies consist: nor of integral parts, as soul and body, of which men consist: nor in a “metaphysical” sense, as of essence and existence, of act and power: nor in a “logical” sense, as of kind and difference, substance and accident; all which would argue imperfection, weakness, and mutability. If God was composed of parts he would not be “eternal”, and absolutely the first Being, since the composing parts would, at least, co-exist with him; besides, the composing parts, in our conception of them, would be prior to the compositum; as the body and soul of man, of which he is composed, are prior to his being a man: and, beside, there must be a composer, who puts the parts together, and therefore must be before what is composed of them: all which is inconsistent with the eternity of God: nor would he be “infinite” and “immense”; for either these parts are finite, or infinite; if finite, they can never compose an infinite Being; and if infinite, there must be more infinities than one, which implies a contradiction: nor would he be “independent”; for what is composed of parts, depends upon those parts, and the union of them, by which it is preserved: nor would he be “immutable”, unalterable, and immortal; since what consists of parts, and depends upon the union of them, is liable to alteration, and to be resolved into those parts again, and so be dissolved and come to destruction. In short, he would not be the most perfect of Beings; for as the more spiritual a being is, the more perfect it is; and so it is, the more simple and uncompounded it is: as even all things in nature are more noble, and more pure, the more free they are from composition and mixture.
Nor is the simplicity of God to be disproved by the Trinity of Persons in the Godhead; for though there are three distinct persons, there is but one nature and essence common to them all, and which is not parted and divided among them, but is jointly and equally possessed by them; nor do these persons really differ from the divine nature and essence, nor from one another, but by their distinct modes of subsisting; so that they only distinguish and modify, but do neither divide nor compose the divine nature: nor is it to be disproved by the decrees of God; the decrees of God are within himself, and, as it is commonly said, whatever is in God, is God, and so are no other than God himself, as to the act of decreeing, though not with respect to the things decreed; and though they are many and various, as to the objects of them, yet not in God, who, by one eternal act, in his infinite mind, has decreed every thing that has been, is, or shall be; and is what Plato means by en kai polla, “one” and “many” in God; one, as to his essence; many, as to the ideas and decrees in it, which many are one: nor is it to be disproved by the attributes of God; for they are no other than God himself, and neither differ from one another, but with respect to their objects, and effect, and in our manner of conception of them; nor from the nature and essence of God; they are himself, and his nature; he is not only eternal, wise, good, loving, &c. but he is eternity itself, wisdom itself, goodness itself, love itself, &c. and these are not parts of his nature, but displays of the same undivided nature, and are different considerations of it, in which we view it; our minds being so weak as not to be able to conceive of God at once and together, and in the gross, but one thing after another, and the same in different lights, that we may better understand it: these several things, called attributes, which are one in God, are predicated of him, and ascribed to him distinctly, for helps to our finite understandings, and for the relief of our minds; and that we, with more facility and ease, might conceive of the nature of God, and take in more of him, as we can by parcels and piecemeals, than in the whole; and so, as a learned Jew observes, all those attributes are only intellectual notions; by which are conceived the perfections that are in the essence of God, but in reality are nothing but his essence; and which attributes will be next considered.” – John Gill, A Body of Doctrinal Divinity, Book 1, Chapter 4, Sec. 3
What Divine Simplicity entails, for someone engaging in theology or arguing contrary to it, is that their formulation must address all of God’s attributes, or at least argue against a conception of God consistent with all of His attributes, even if they are not specifically addressed. If they do not do so accurately, they are guilty of arguing for or against an incomplete picture of God. This is not something new, but long recognized in theology. What is unfortunate is that believers so often fail to object to these malformed arguments from the grounds of proper theology – which should be the basis of our philosophy.