The nature of God is paramount in apologetics, so today we will continue to explore what that nature entails. The article below explains the simplicity of God:
Simplicity (Indivisibility). Since God is not composed in his Being, but is Pure Existence, Pure Actuality with no potentiality; it follows that he is simple and indivisible. A Being that by nature is not composed cannot be decomposed. One that has no parts cannot be torn apart. Hence, God has absolute simplicity with no possibility of being divided. He is literally indivisible.
Likewise, a God of Pure Actuality with no potentiality cannot be divided. For if it were divisible, then it would have to have the potential to be divided. But Pure Actuality has no potentiality in its Being whatsoever. Hence, it must be absolutely simple or indivisible.
God’s indivisibility follows also from his immutability (see below). For if God could be divided, he could change. But God is unchangeable by nature. Thus he cannot be divided. He must be absolutely simple in his nature.
Geisler, N. L. (1999). In Baker encyclopedia of Christian apologetics (pp. 282–283). Grand Rapids, MI: Baker Books.