I AM THE LORD AND I DO NOT CHANGE
God is bold enough to declare this simple statement, I AM. Together with this statement that "I do not change". This declaration is a majestic proclamation that God, whoever he is, does not change. If he is good, then that goodness never changes. Whatever his attributes, like mercy and justice, those do not change. Ever. This is the Doctrine of Immutability. God is Lord and he does not change.
If one is to believe in a God, it is essential that he is immutable. Faith is based on the trust that God says who he is. God must therefore be immutable for the Bible to be true. For the prophecies and the promises to be of any value, God must be the ultimate being. He must be who he has always been. Otherwise everything that faith calls for needs to change.
If God makes a promise then he must be capable of fulfilling his word. Also he must be trusted to deliver on his promise. Trusting God is by definition believing in his unchanging nature.
Changing is something humans have to do to regain their status before the fall of man. God is perfect, therefore does not need to change. Immutability and perfection are connected. If one is perfect then change is not required. The Bible teaches that God never changes. The promises are unchanging. God boldly declared in Malachi 3:6 that he is unchanging. The immutability of God is thus certain and an essential part of the doctrine of Christianity.