To say that God is immutable is to say that He does not change. In Malachi 3:6, God says, "I, the Lord do not change."
God has neither evolved, grown, nor improved.
God does not deteriorate.
God does not develop.
God does not mature.
God does not ripen.
God needs no upgrades or tune-ups.
If God could change for the better, He would not currently be the best possible being in the universe. If God could change for the worse, what kind of God could He end up being?
Affirming God's immutability is basically affirming that He is God. He has always, is, and will always be consistent with Himself.
In Knowing God, JI Packer list some ways in which God is immutable:
1. God’s life does not change. He is immutable in His essence (Psalm 102:25-27).
2. God’s character does not change. He is immutable in His attributes (James 1:17).
3. God’s truth does not change. He is immutable in His communication (Isaiah 40:8).
4. God’s purposes do not change. He is immutable in His plans (Psalm 33:10-11; Isaiah 14:24; Num. 23:19; 1 Sam. 15:28-29).
5. God’s Son does not change. He is immutable in His saving power (Hebrews 13:8).
God is very dependable. In this rapidly changing world, God is constant, immovable, and steadfast. God's immutability is the foundation for all our confidence in God's promises and purposes.