To say that God’s immutable means that he’s unchanging in his being, perfections, purposes and promises. So, his being — his nature, his essence; his perfections — the degree to which he possesses those characteristics; his purposes — what he has determined to do; and his promises — what he’s told us he’ll do. So, God’s unchanging in those ways. That’s not to say that God doesn’t relate to us in a dynamic, relational, personal way. So, he listens to our prayers, he grieves over our sin, he delights in our faithfulness. And so, it’s been said God is essentially immutable, but relationally mutable. There’s a degree to which he adapts what he’s doing to our relationship with him, while at the same time maintaining his essential attributes. [Dr. K. Erik Thoennes]

The biblical concept of divine immutability doesn’t imply that God is inactive. In biblical terms, an inactive god is a worthless idol. But the God of Scripture is constantly interacting with his creation in real and meaningful ways.

Traditional Christian theology has rightly insisted that there are three critical aspects to God’s immutability. First, God’s eternal counsel, or ultimate plan for history, is immutable. Although different Christian traditions understand God’s eternal plan differently, we should all agree that everything God has done, is doing, and will do is part of a unified plan. God knows everything, and he is using that knowledge to direct history toward the ends for which he created it. As God said in Isaiah 46:10:

I make known the end from the beginning, from ancient times, what is still to come. I say: My purpose will stand, and I will do all that I please.

And as Paul explained in Ephesians 1:4, 11:

He chose us in [Christ] before the creation of the world… In [Christ] we were also chosen, having been predestined according to the plan of him who works out everything in conformity with the purpose of his will.

Paul made it clear that God has a plan that covers everything. And this plan has existed since the time when he “chose” or predestined believers to salvation. Of course, different traditions interpret the concept of predestination differently. But what is beyond question is that God predestined before he even created the world. Predestination was just part of his eternal counsel. And this counsel is immutable because God causes everything to conform to it.

In the second place, God is also immutable in his character. His essence, persons and attributes never change. Now to be sure, God reveals different aspects of his character more prominently at some times than others. Sometimes he displays his mercy, sometimes his wrath. Sometimes he reveals his omniscience and other times he hides it. But the full range of his attributes — his eternal nature — always stays the same. That’s why in James 1:17, James referred to God as:

The Father of the heavenly lights, who does not change like shifting shadows.

In the third place, God is unchanging or immutable in his covenant promises. God will fulfill everything he has ever sworn in covenant. When God promises, his word is immutable. Otherwise, he’s free to change his mind. Consider Genesis 15 where God said he would make Abraham’s offspring as numerous as the stars. Abraham was thankful for this offer, but he still asked God to make the blessing certain. So, God responded by making a covenant with him.

Every revelation of God in Scripture assumes that God will keep his covenant and his covenant promises. The original audiences were supposed to understand every passage of Scripture in this light, and modern audiences should do the same. We should have absolute confidence in God’s unchanging promises. And his offers and threats should motivate us to obedience.