“God is and remains the same. Everything changes, but He remains standing.
He remains who He is (Ps. 102:26–28). He is YHWH, He who is and ever remains Himself.
He is the first and with the last He is still the same God (Isa. 41:4; 43:10; 46:4; 48:12). He is who He is (Deut. 32:39; cf. John 8:58; Heb. 13:8), the incorruptible who alone has immortality, and is always the same (Rom. 1:23; 1 Tim. 1:17; 6:16; Heb. 1:11–12).
Unchangeable in His existence and being, He is so also in His thought and will, in all His plans and decisions. He is not a human that He should lie or repent.
What He says, He will do (Num. 15:28; 1 Sam. 15:29). His gifts (charismata) and calling are irrevocable (Rom. 11:29). He does not reject his people (Rom. 11:1).
He completes what He has begun (Ps. 138:8; Phil. 1:6). In a word, He, YHWH, does not change (Mal. 3:6).
In Him there is ‘no variation or shadow due to change’ (James 1:17).” On this foundation Christian theology constructed its doctrine of divine immutability…
If God were not immutable, He would not be God. His name is ‘being,’ and this name is “an unalterable name.” All that changes ceases to be what it was. But true being belongs to Him who does not change.
That which truly is remains. That which changes was something and will be something but is not anything because it is mutable.
But God who is cannot change, for every change would diminish His being. Furthermore, God is as immutable in His knowing, willing, and decreeing as He is in His being.
The essence of God by which He is what He is, possesses nothing changeable, neither in eternity, nor in truthfulness, nor in will. As He is, so He knows and wills—immutably.
Augustine wrote, ‘For even as You totally are, so do You alone totally know, for You immutably are, and You know immutably, and You will immutably. Your essence knows and wills immutably, and Your knowledge is and wills immutably, and Your will is and knows immutably.’ (Confessions, XIII, 16)
Neither creation, nor revelation, nor incarnation (affects, etc.) brought about any change in God. No new plan ever arose in God. In God there was always one single immutable will.”
–Herman Bavinck, Reformed Dogmatics: God and Creation, Ed. John Bolt, and Trans. John Vriend (vol. 2; Grand Rapids, MI: Baker Academic, 2004), 2: 153-154.