The nature of God is paramount in apologetics, so today we will continue to explore what that nature entails. The article below explains the immutability of God:
Immutability (Unchangeability). In his epic Summa Theologica (1a.9.1), Aquinas offers three basic arguments in favor of God’s unchangeability. The first argument is passed on the fact that a God of Pure Actuality (“I-Am-ness”) has no potentiality. It follows, therefore, that God cannot change (Exod. 3:14). Whatever changes has to have the potential to change. But as pure Actuality, God has no potential, so he cannot change.
The second argument for God’s immutability follows from his simplicity. Everything that changes is composed of what changes and what does not change. God cannot change because an absolutely simple being has no composition. If everything about a being changed, then it would be an entirely new being. In fact, it would not be change but annihilation of one thing and a creation of something entirely new. Now if, in every change in a being something remains the same and something does not, then it must be composed of these two elements. So an absolutely simple Being with no composition cannot change.
The third argument for God’s unchangeability argues from his absolute perfection. Whatever changes acquires something new. But God cannot acquire anything new, since he could not be better or more complete. Therefore, God cannot change. If he did, he would not be God for he would have lacked some perfection.
Aquinas also argues that God alone is immutable (Summa Theologica, 1a.9.2). All creatures exist only because of the will of the Creator. His power brought them into existence, and it is his power that keeps them in existence. Therefore, if he withdrew his power they would cease to exist. Whatever can cease to exist is not immutable. Therefore, God alone is immutable; everything else could cease to exist.
Geisler, N. L. (1999). In Baker encyclopedia of Christian apologetics (p. 283). Grand Rapids, MI: Baker Books.