When theologians speak of the simplicity of God, they are not saying that God is easy to understand or denying that God is an infinitely complex being who transcends our human reasoning. When speaking of God as simple, the term is being used in contrast to being composite or made up of many parts. The simplicity of God means that God is his attributes, not that God is made up of his attributes. That means the attributes of God are his perfections and are therefore not subject to change. God is infinite and perfect in all his attributes. His attributes do not exist outside himself, but he is his attributes. The love of God is not one part of God or something that he has, but defines who he intrinsically and eternally is. God defines love because love is essential to his nature. Love is who he is but love is not all that he is. Kevin DeYoung defines simplicity this way: “Simple, as a divine attribute, is the opposite of compound. The simplicity of God means God is not made up of his attributes. He does not consist of goodness, mercy, justice, and power. He is goodness, mercy, justice, and power. Every attribute of God is identical with his essence.” James Dolezal defines simplicity as “nothing that is not God can account for the being of God, and so all that is in God must be God.”
That means God has no accidental properties. An accidental property is something that is unessential to the nature of a being. Every attribute is essential to and defines who God is. That means God is not sovereign over his own nature as if he could change it, but rather, his nature is who he is and he is not subject to the possibility of change. To speak of God as having parts would indicate dependence and finitude as opposed to independence and infinity. When the Westminster Confession of Faith speaks of God as “without parts or passions” it is respectively affirming God’s simplicity and impassibility. There is a correlation between the aseity or independence of God, impassibility, simplicity, eternality, immutability, and transcendence. Because God is transcendent and independent of time and creation, he cannot be acted upon and therefore cannot suffer and must be impassible. To suffer is to be acted upon by an outside agent who inflicts a change in our state of being. But God is most absolute, most loving, and most holy. That means God is a pure actuality who is not coming into being or undergoing change, but eternally exists as an unchanging being (Exo 3:14). God is not constantly taking in new knowledge because he transcends time and space as one who is perfect in knowledge. God’s knowledge of us is productive rather than receptive. His will for us to have existence is the basis for his knowledge of us.
Divine simplicity also proves monotheism or the belief that there is only one God. God is not a species of divinity as if God or deity could be divided into parts or subcategories. God is without spatial or temporal parts. God is good in virtue of himself and defines what good is because all that is good is rooted in God’s nature. The basis for what is right and what is wrong is the holy unchanging character of God. That is why the moral law cannot be subject to change. It is always wrong to lie because God is truth and his nature cannot change. His love defines what true love is and his righteousness defines how we are to live. Without God, we have no basis for universal objective morality that is binding on all men at all times in all places. God is not partly love and partly holy or is more loving than holy, but is infinitely loving and infinitely holy. That is why his perfect law had to be satisfied through the passive and active obedience of Christ which is imputed to us in justification. The glory and holiness of God describe and summarize all of God’s attributes which is the essence of his infinite worth and uniqueness.
When we look at the biblical basis for divine simplicity, every verse which speaks of God with a verb of being lends support for the doctrine. Remember that the name Yahweh is etymologically related to the Hebrew verb hayah or “to be.” God is the one who was, who is, and who is to come (Rev 1:4). Scripture says, “the Lord is slow to anger” (Num 14:18), “the Lord is righteous” (2 Chron 12:6), “our God is holy” (Psa 99:9), “the Lord is good” (Psa 100:5), “the Lord is merciful and gracious” (Psa 103:8), “God is spirit” (John 4:24), “the Lord is faithful” (2 Thess 3:3), “the Lord is compassionate and merciful” (James 5:11), “God is light” (1 John 1:5), “he is righteous” (1 John 2:29), “God is love” (1 John 4:8), God “is true” (1 John 5:20). For a useful discussion of divine simplicity, I recommend this talk by James Dolezal.