What does it mean that God is immutable?
Here's the answer:
Immutable (im-ute-able) is a big word. When we say God is immutable, we mean that He doesn’t ever change.
God is perfect and He won’t ever stop being perfect. It’s impossible to get better than perfect, and He certainly won’t get worse. He will always and forever be who He is. God says it Himself in the Bible: "I am the LORD. I do not change..." (Malachi 3:6). God is the same as He was way back when the earth was created, and He will be the same in the future. When we read about God in the Bible, we can know that He is still exactly the same now as He was when the Bible was written.
Because God is immutable, He won’t change the way He does things. He will always do what’s right, and He will always do what’s best for us. And because God is immutable, we can trust in Him completely. We don’t have to be afraid that He’ll break His promises to us, or that we’ll wake up one day and find that He is different.
"God isn’t a mere human...He doesn’t change his mind. He speaks, and then he acts. He makes a promise, and then he keeps it" (Numbers 23:19).
"Before you created the whole world and the mountains were made, from the beginning to the end you are God" (Psalm 90:2).