This is one of the Divine perfections which is not sufficiently pondered. It is one of the excellencies of the Creator which distinguishes Him from all His creatures. God is perpetually the same: subject to no change in His being, attributes, or determinations. Therefore God is compared to a rock (Deut 32:4, etc.) which remains immovable, when the entire ocean surrounding it is continually in a fluctuating state; even so, though all creatures are subject to change, God is immutable. Because God has no beginning and no ending, He can know no change. He is everlastingly "the Father of lights, with whom is no variableness, neither shadow of turning" (Jas. 1:17).
First, God is immutable in His essence. His nature and being are infinite, and so, subject to no mutations. There never was a time when He was not; there never will come a time when He shall cease to be. God has neither evolved, grown, nor improved. All that He is today, He has ever been, and ever will be. "I am the Lord, I change not" (Mal. 3:6) is His own unqualified affirmation. He cannot change for the better, for He is already perfect; and being perfect, He cannot change for the worse. Altogether unaffected by anything outside Himself, improvement or deterioration is impossible. He is perpetually the same. He only can say, "I am that I am" (Ex. 3:14). He is altogether uninfluenced by the flight of time. There is no wrinkle upon the brow of eternity. Therefore His power can never diminish nor His glory ever fade.
Secondly, God is immutable in His attributes. Whatever the attributes of God were before the universe was called into existence, they are precisely the same now, and will remain so forever. Necessarily so; for they are the very perfections, the essential qualities of His being. Semper idem (always the same) is written across every one of them. His power is unabated, His wisdom undiminished, His holiness unsullied. The attributes of God can no more change than Deity can cease to be. His veracity is immutable, for His Word is "forever settled in heaven" (Ps. 119:89). His love is eternal: "I have loved thee with an everlasting love" (Jer. 31:3) and "Having loved His own which were in the world, He loved them unto the end" (John 13:1). His mercy ceases not, for it is "everlasting" (Ps. 100:5).
Thirdly, God is immutable in His counsel. His will never varies. Perhaps some are ready to object that we ought to read the following: "And it repented the Lord that He had made man" (Gen. 6:6). Our first reply is, Then do the Scriptures contradict themselves? No, that cannot be. Numbers 23:19 is plain enough: "God is not a man, that He should lie; neither the son of man, that He should repent." So also in 1 Samuel 15:19, "The strength of Israel will not lie nor repent: for He is not a man, that He should repent." The explanation is very simple. When speaking of Himself. God frequently accommodates His language to our limited capacities. He describes Himself as clothed with bodily members, as eyes, ears, hands, etc. He speaks of Himself as "waking" (Ps. 78:65), as "rising early" (Jer. 7:13); yet He neither slumbers nor sleeps. When He institutes a change in His dealings with men, He describes His course of conduct as "repenting."
Yes, God is immutable in His counsel. "The gifts and calling of God are [irrevocable]" (Rom. 11:29). It must be so, for "He is in one mind, and who can turn Him? and what His soul desireth, even that He doeth" (Job 23:13). Change and decay in all around we see, may He who changeth not abide with thee. God’s purpose never alters. One of two things causes a man to change his mind and reverse his plans: want of foresight to anticipate everything, or lack of power to execute them. But as God is both omniscient and omnipotent there is never any need for Him to revise His decrees. No. "The counsel of the Lord standeth forever, the thoughts of His heart to all generations" (Ps. 33:11). Therefore do we read of "the immutability of His counsel" (Heb. 6:17).
Herein we may perceive the infinite distance which separates the highest creature from the Creator. Creaturehood and mutability are correlative terms. If the creature was not mutable by nature, it would not be a creature; it would be God. By nature we tend to nothing, as we came from nothing. Nothing stays our annihilation but the will and sustaining power of God. None can sustain himself a single moment. We are entirely dependent on the Creator for every breath we draw. We gladly own with the Psalmist Thou "holdest our soul in life" (Ps. 66:9). The realization of this ought to make us lie down under a sense of our own nothingness in the presence of Him "in Whom we live and move, and have our being" (Acts 17:28).
As fallen creatures we are not only mutable, but everything in us is opposed to God. As such we are "wandering stars" (Jude 13), out of our proper orbit. The wicked are "like the troubled sea, when it cannot rest" (Isa. 57:20). Fallen man is inconstant. The words of Jacob concerning Reuben apply with full force to all of Adam’s descendants: "unstable as water" (Gen. 49:4). Thus it is not only a mark of piety, but also the part of wisdom to heed that injunction, "cease ye from man" (Isa. 2:22). No human being is to be depended on. "Put not your trust in princes, in the son of man, in whom is no help" (Ps. 146:3). If I disobey God, then I deserve to be deceived and disappointed by my fellows. People who like you today, may hate you tomorrow. The multitude who cried "Hosanna to the Son of David," speedily changed to "Away with Him, Crucify Him."
Herein is solid comfort. Human nature cannot be relied upon; but God can! However unstable I may be, however fickle my friends may prove, God changes not. If He varied as we do, if He willed one thing today and another tomorrow, if He were controlled by caprice, who could confide in Him? But, all praise to His glorious name, He is ever the same. His purpose is fixed, His will stable, His word is sure. Here then is a rock on which we may fix our feet, while the mighty torrent is sweeping away everything around us. The permanence of God’s character guarantees the fulfillment of His promises: "For the mountains shall depart, and the hills be removed; but my kindness shall not depart from thee, neither shall the covenant of My peace be removed, saith the Lord that hath mercy on thee" (Isa. 54:10).
(Accessed from http://www.pbministries.org/books/pink/Attributes/attrib_07.htm )