To say that God is immutable is to say that He never differs from Himself. The concept of a growing or developing God is not found in the Scriptures. It seems to me impossible to think of God as varying from Himself in any way. Here is why:
For a moral being to change it would be necessary that the change be in one of three directions. He must go from better to worse or from worse to better; or, granted that the moral quality remain stable, he must change within himself, as from miniature to mature or from one order of being to another. It should be clear that God can move in none of these directions. His perfections forever rule out any such possibility.
God cannot change for the better. Since He is perfectly holy, He has never been less holy than He is now and can never be holier than He is and has always been. Neither can God change for the worse. Any deterioration within the unspeakably holy nature of God is impossible. Indeed I believe it impossible even to think of such a thing, for the moment we attempt to do so, the object about which we are thinking is no longer God but something else and someone less than He. The one of whom we are thinking may be a great and awesome creature, but because he is a creature he cannot be the self-existent Creator.
As there can be no mutation in the moral character of God, so there can be none within the divine essence. The being of God is unique in the only proper meaning of that word; that is, His being is other than and different from all other beings. We have seen how God differs from His creatures in being self-existent, selfsufficient, and eternal. By virtue of these attributes God is God and not some other being. One who can suffer any slightest degree of change is neither self-existent, self-sufficient, nor eternal, and so is not God.
Only a being composed of parts may change, for change is basically a shift in the relation of the parts of a whole or the admission of some foreign element into the original composition. Since God is self-existent, He is not composed. There are in Him no parts to be altered. And since He is self-sufficient, nothing can enter His being from without.
“Whatever is composed of parts,” says Anselm, “is not altogether one, but is in some sort plural, and diverse from itself, and either in fact or in concept is capable of dissolution. But these things are alien to Thee, than whom nothing better can be conceived of. Hence, there are no parts in Thee Lord., nor art Thou more than one. But Thou art so truly a unitary being, and so identical with Thyself, that in no respect art Thou unlike Thyself, rather Thou art unity itself, indivisible by any conception.”
“All that God is He has always been, and all that He has been and is He will ever be.” – A.W. Tozer