The unchanging God
The Westminster Catechism, in presenting a brief definition of God, succinctly says that He is infinite, eternal and unchangeable in His Being.
God's nature or character is immutable, that is, it has never worsened or bettered - it remains the same. God does not change, indeed, he cannot change, both in his being (ontologically) and in his will (decretally).
The constancy of his character, his nature or his essential being is brought out. Not only in his character but also in his design and purpose, so much so that Scripture affirms the eternal counsel of God: What God is doing in history since the creation to its consummation is according to his infallible and unchanging plan.
This constancy guarantees that He remains always one and the same true God, faithful to himself (in Trinitarian relationships), to his decrees (secret in himself but not being divulged and executed in history) and his works (in relation with angels and men.
Scripture is replete with affirmations concerning the unchanging Triune God: some explicit ones are the following:
Numbers 23:19 - “God is not a man, that He should lie, nor a son of man, that He should repent.”
1 Samuel 15:29 - "And also the Strength of Israel will not lie nor repent: for he is not a man, that he should repent."
Psalms 102:26 - "They shall perish, but thou shalt endure; yea, all of them shall wax old like a garment; as a vesture shalt thou change them, and they shall be changed."
Malachi 3:6 - "For I am the Lord, I change not; therefore ye sons of Jacob are not consumed."
2 Timothy 2:13 - "If we believe not, yet he abideth faithful: he cannot deny himself."
Hebrews 6:17,18 - "Wherein God, willing more abundantly to shew unto the heirs of promise the immutability of his counsel, confirmed it by an oath: that by two immutable things, in which it was impossible for God to lie, we might have a strong consolation, who have fled for refuge to lay hold upon the hope set before us."
James 1:17 - "Every good gift and every perfect gift is from above, and cometh down from the Father of lights, with whom is no variableness, neither shadow of turning."
Such passages serve that we may worship God aright; they are also for our comfort.
God's immutability defines all his other attributes: he is immutably wise, he cannot but be merciful, good, and gracious. The same may be said about his knowledge: God does not need to gain knowledge; he knows all things, eternally and immutably so.
Now, in his communication to us, God was pleased to give promises, and most importantly, to enter into covenant with his elect through their appointed Mediator, Jesus Christ. God has bound himself to protect, to cherish, to sanctify, to deliver his people, and finally to bring them into glory.
The importance of his immutability in knowledge at once becomes apparent, for if He forgets or lets some knowledge about his children slip by, how can He fulfil what He has promised? When they are tempted, how can He deliver them?
He cannot prove to be trustworthy if his knowledge is slack or inconstant.
God's will and purposes
Nobody can exercise his will or fulfil his purpose without the aid of knowledge. Knowledge is primary and indispensable for rational beings, and all the more so for the supremely rational God, the source of all logic, mind, and understanding.
God's secret will is in himself: He alone knows it, and He knows it eternally and infallibly so, to such an extent that the Scripture affirms that God works all things after the counsel of his own will (Ephesians 1:11). But he cannot do so without knowledge, and He needs to know all the details, if all things are to be done according to his unchanging will.
Again, God's revealed will (what he said men should do, how they are to behave) implies that He is unchanging in his knowledge of all his creatures. Otherwise how can He be their righteous Judge and Rewarder if He lacks information about their goings in and goings out?
The importance of this doctrine
The revelation of God's name, his character, his attributes and all that pertain to him, directly or indirectly, affects our worship of him. A false or incorrect notion of God is a subtle form of idolatry. The church must be well-grounded in this: it is the people who know their God that will do exploits, and be strong.
Now Scripture gives us no allowance to diminish or in some way tarnish the attributes of God, which are all perfect and infinite. Since God is knowing, and God is eternal, He must have known from eternity, and if He has known from eternity, then his knowledge is necessarily unchangeable. These are all interrelated and mutually defining.
If all men are naked and open before Him with whom we have to do (Hebrews 4:13; 2 Chronicles 16:9; Job 26:6), then God did not begin someday to gain knowledge about us; He knew us from the beginning, indeed from eternity past (Psalms 90).
This is important because it elevates our thoughts concerning God; we would not be tempted to lower him down to our level, but indeed worship him as the supreme and only God. He needs not to be instructed (Isaiah 40:12ff.): even when we pray He knows already what we will ask. So, we are not to pray thinking that we will convince God to do our will; rather prayer is to be considered as the God-appointed means for God to do His own will concerning us and others.
God’s glory is complete
God is glorious in and of himself, not needing anything or anyone to enhance or add to his intrinsic majesty: see Isaiah 6:3; Psalms 24:7-10; Acts 7:2. Glory is inherent to his very nature. Even without creation, his glory would not be less that what it is.
God's glory is the sum total of all his attributes or characteristics (cf. Exodus 33:17-19).
It can be seen and appreciated by men on earth: "We have beheld his glory..." (John 1:14); and all the more so in heaven: "That they may behold my glory, glory which thou hast given me before the foundation of the world" (John 17:24).
Men can only acknowledge and confess God's glory, but whether they do so or not God is still all-glorious. We cannot add or diminish to his attributes. We may and are indeed bound to glorify (adorn) his doctrine (truth), and when we do so we do this for our own good.
By contrast an earthly monarch does not possess intrinsic glory: it is given to him (by the sceptre, the throne, the royal garments, and so on).
According to Paul, though, God is not served with human hands, as if God lacks anything, since he gives to all life and breath and all things. But the creature is bound and morally obliged (as a creature) to declare and give glory to God, that is, acknowledge God for who He really is (1 Chronicles 16:24,27,28; 29:11-13).
Furthermore God cannot and will not give his glory to another; it belongs to him essentially (Isaiah 48:11). But his people willingly exhibit and make known his glory in the eyes of men (Titus 2:10; Philippians 1:20; 1 Timothy 1:17; 6:15-16; Jude 24). Even the created order glorifies God in its own way (Psalms 19:1).
But the whole point is that God is glorious and blessed in himself (James 2:1). Nothing and nobody can enhance his intrinsic glory or detract from it. His glory is manifested over, in and through his creatures.
God is infinite, therefore immutable
Finiteness and infiniteness are mutually exclusive categories. A finite being, therefore, can never become infinite, for in multiplying a finite dimension by finiteness the result would still be finite. A finite, limited thing must necessarily have a beginning; it must be created. So it must also be mutable, for since there was a time when it was not, then there was a radical change from non-being to being.
By contrast, God is immutable for He never came into being. He is Being, and the source of all other beings. He is always the same in his being: then He must be infinite. If not infinite, then finite; but finiteness spells and carries with it the idea of change (from non-being to being, at least).
Infiniteness and immutability in God are mutually supportive and imply each other. An infinite and changing God is inconceivable; indeed it is a contradiction in terms.
Objection: God changes because he repents
How does the idiom concerning God repenting (cf. Exodus 32:9,10; 1 Samuel 15:11) square with the unalterable fixity of his eternal decree?
When the Bible seems to suggest that God did in fact alter his course of action away from a previously declared course of action, one should understand that his "new course" is only his settled, immutably certain response in keeping with the principles of conduct respecting himself which He himself declared (Jeremiah 18:7-10). In other words, God always acts the same way toward moral evil and the same way toward moral good. In his every reaction to men's responses to him, the immutable moral fixity of his character is evident. If people alter their relations to him, He will always respond in a manner consistent with his immutably holy character (see Jonah 3:3-10; also read Psalms 18:25-27).
The Bible takes seriously the character of God who as holy can never approve of evil and who must always recoil (repent) against it even though he decreed its existence; who as just must always approve of obedience, pronounce it good, and rejoice over it even though, where it actually exists in the creature, he is the ultimate author of it; and who, simply because he is good, must always respond to the sinner's evil with grief and to the sinner's repentance with delight.
Obejction: God grieves in His heart, therefore He is mutable
Passages like Genesis 6:5-7, speaking about God being grieved in his heart, are regularly misused by some who entertain false notions about God's immutability.
First of all, such language is anthropopathetic, that is, human passions and emotions are ascribed to God for the sake of our finite understanding, in the same way as bodily parts are ascribed to him (anthropomorphisms).
God, being not only infinitely holy but also the God of infinite goodness and compassion, in reaction to the evil of those who wilfully refuse to obey him, is said (and rightly so) to be grieved that He had made them. In fact it would be strange if we did not hear him say that their sin and evil were a source of great grief to him. He takes no pleasure in wickedness or in the destiny of lost people (Ezekiel 33:11).
God cannot look upon man's sin with acceptance (Habakkuk 1:13). The creature's obedience always brings him joy; the creature's sin always grieves him, even to the point that He poignantly declares that He regrets that He made those who disobey him.
All this further reinforces his immutability (in that He always acts and reacts the same way morally), rather than puts it in doubt.