a) It maintains the proper distinction between Creator and creature. The Dutch theologian Herman Bavinck said, ‘The doctrine of God’s immutability is of the highest significance for religion. The contrast between being and becoming marks the difference between the Creator and the creature. Every creature is continually becoming, changeable, constantly striving, seeks rest and satisfaction, and finds rest in God, in Him alone, for only He is pure being and no becoming. Hence, in Scripture God is often called the ‘Rock.’ That the doctrine of God’s immutability leads to a proper distinction between the Creator and the creature means that when we leave immutability behind, or distort it to any degree we blur the line between Creator and creature. When that line is blurred, it ultimately leads to an elevated view of man which always leads to a low view of God. Nothing good comes from having a low view of God. If we have a low view of God we’ll have a low view of God’s Word, God’s Church, and God’s ways/wisdom. Again, safeguard yourself from error by embracing a big view of God and you’ll save yourself much trouble.
b) It gives us hope. Malachi 3:6 says, ‘For I the Lord do not change, therefore you, O children of Jacob are not consumed.’ The reason we’re not consumed is because God does not change. God says His immutability is the ground of our hope because regardless of what is going on in our lives – when we draw near to God we’ll always be able to find Him because He always the same. Just imagine if God did change, taking our former reasoning that ‘to change’ means one becomes either better or worse. This leads to awful implications with God. If God were to change for the better than He would not have been the best possible being when we first trusted Him. And if He could become better how He is the best possible being now? If God was changing for the worse, than what would that even mean? He could become evil, or wholly evil. How could we trust a God who could change? If His eternal will, purposes, and promises were always up for grabs there’s no reason to trust the Bible either, and eventually there would be no reason to think that the blood of Jesus saves us, cleanses us, and washes us white as snow, because God may change His mind. Or what if His power changed making Him unable to save us (even if He wanted to) or come through on any of His promises?
Wayne Grudem has a helpful comment here: ‘A little reflection shows how absolutely important the doctrine of God’s immutability is. If God can change, the whole basis of our faith begins to fall apart. God is infinitely worthy of our trust BECAUSE He is absolutely and eternally unchanging in His being, perfections, purposes, and promises.’