"I, the Lord, do not change." So God declares in Malachi 3:6. We find similar statements throughout Scripture, and some of the most constant refrains in Scripture are that God remains faithful to His Word, that He keeps His promises, that His character neither falters nor shifts. Most of the time, there is a very specific point being made. These statements serve as reminders for hope, as high standards for shame, or as aids to faith. The primary point of them usually seems straightforward. You can trust God, or hope in Him, or wait for Him, because He is who He has always been, will do what He has always done, and will stand by what He has always said. So God's faithfulness and constancy are major themes in Scripture.
However, there are some bits of Scripture that point beyond this. Recall that God is the one who created all things. He is the one who comes from eternity and shall outlast all of the ages. He is, as we saw before, completely perfect, completely good, without rival. No one can overpower Him, transcend His wisdom, or do anything He does now know or foresee. All of these point far beyond a God with merely a steady personality, or even unshakeable virtue. The God who Himself created all everything involving movement, time, metamorphosis, and all kinds of change cannot be changed by any of them. Moreover, since He is already always fully active in all of His infinite goodness, there is nowhere to go for Him to change Himself in any way. God is fully and completely changless. He is immutable.
Now, we should be very clear here. God never changes. No part of Him changes, because He has no parts. He is yesterday, today, and forever always as He is. It's not only that who God is doesn't change, not even that what He is doesn't change, but that His whole and simple self remains perfectly identical beyond every time and way. Take this as strongly as you need to, because there is a question this raises sometimes.
If God is absolutely and completely beyond change of any kind, is God not dead? Is not change required to live and move and act? If God is unchangeable, isn't He lifeless and "stuck," an eternal stick in the mud? By no means! As we said before, God has not wasted potential. That God is fully and completely active in all His power, glory, might, and majesty is yet another fact about Him that never changes. God is not like a corpse who stops moving and changing because it has no life left to move. That is a condition of living in time, where we never work out our full potential except over time, because we are limited in many ways. God, who, as we saw before, has no limits, is able to be fully effective in His eternal, single, changless existence. As an analogy, consider many kinds of earthly work. If a hammer is able to do in seconds what it would take a woodpecker days to do, which is more powerful? The more power something has, the more it can do in less time. God, being infinitely powerful, requires no time at all to perform everything He does, and so can be fully alive without ever having to change.
It may be possible as well to ask if Scripture doesn't contradict this. Does not the Bible mention God changing His mind, and other such things? Well, yes it does. But it is vital to ask a simple question: does the Bible ever make it look like God was caught off guard by anything, ever suprised by any action anyone ever took? No! He is the one who knows all things, future and past and present. If God "changes His mind," we can only understand this as an oversimplified analogy for the benefit of our quite limited minds. What has changed in these cases is not really the mind of God Himself, but the earthly situation He acts into, and this was always His plan from the beginning.
To summarize, then, God is wholly immutable. Nothing in Him changes, for His has no parts. Nor is there any need for Him to change, the One who lives in eternity able to do all things in unbridled power without even a moment of time. He always was, is, and will be, the same God now and forever, who wheels the universe around Himself and reorders it so that He might dispense in His immutable providence everything which rightly belongs in our world. The I AM is.