What is divine simplicity? I have heard it but, am not familiar with what it exactly is.
2. THE UNITAS SIMPLICITATIS.
While the unity discussed in the preceding sets God apart from other beings, the perfection now under consideration is expressive of the inner and qualitative unity of the Divine Being. When we speak of the simplicity of God, we use the term to describe the state or quality of being simple, the condition of being free from division into parts, and therefore from compositeness. It means that God is not composite and is not susceptible of division in any sense of the word. This implies among other things that the three Persons in the Godhead are not so many parts of which the Divine essence is composed, that God’s essence and perfections are not distinct, and that the attributes are not superadded to His essence. Since the two are one, the Bible can speak of God as light and life, as righteousness and love, thus identifying Him with His perfections. The simplicity of God follows from some of His other perfections; from His Self-existence, which excludes the idea that something preceded Him, as in the case of compounds; and from His immutability, which could not be predicated of His nature, if it were made up of parts. This perfection was disputed during the Middle Ages, and was denied by Socinians and Arminians. Scripture does not explicitly assert it, but implies it where it speaks of God as righteousness, truth, wisdom, light, life, love, and so on, and thus indicates that each of these properties, because of their absolute perfection, is identical with His Being. In recent works on theology the simplicity of God is seldom mentioned. Many theologians positively deny it, either because it is regarded as a purely metaphysical abstraction, or because, in their estimation, it conflicts with the doctrine of the Trinity. Dabney believes that there is no composition in the substance of God, but denies that in Him substance and attributes are one and the same. He claims that God is no more simple in that respect than finite spirits.
Berkhof, L. (1938). Systematic theology (p. 62). Grand Rapids, MI: Wm. B. Eerdmans publishing co.