God is immutable because time-space does not act upon God. God is not a mere participant in the cosmos, allowing events to form or shape Him. He is not passive: world history does not imprint itself on Him and change Him. This is what is meant by God’s impassibility. It is not that God has no affections. It is that God is not a passenger in the train-car of time, reacting to what life throws at Him. Instead, all of cosmic history is encompassed in the being of God, and all of God’s responses to all events are in His eternal Now. I AM THAT I AM.
The covenant name of God communicates profound depth with a two-word simplicity: I AM. The hallowed Name of God contains a wealth of truth, if we will stop to peer into its depths, and not merely notice its surface.
I AM THAT I AM reveals a God who is immutable: unchanging and unchangeable. To say I AM THAT I AM is to say, I WILL ALWAYS BE WHAT I AM, or I AM ALL THAT I WILL EVER BE, or even I AM WHAT I HAVE ALWAYS BEEN. God has never, can never, and will never change, in the slightest degree. “For I am the LORD, I do not change” (Malachi 3:6)
Change belongs to finite beings. Our finitude means we are capable of growth or diminution. We can improve, or decline. We can develop or decay. We are not what we will be. We are not what we were. We can never say, I am what I will be, or I am what I have always been. We retain a sense of continuing identity across time, but we change continually.
Change also belongs to finite beings who experience time sequentially. For us, time is a stream of events coming to us from the future and then receding into the past. Each moment of time changes us in slight ways: we age, we learn, we react. Each moment is new and unknown to us until it arrives.
Finitude and sequential time are not part of God’s existence. God is unchanging because His being is infinite in all perfections. Were God able to grow, learn, or improve, it would imply that His being still fell short of infinite perfection, however little. Were God able to suffer, forget, or be weary, it would imply His being could experience decay or regression from perfection. But to move towards or away from perfection is to change, and God cannot and does not change.