Summa Theologica, by St. Thomas Aquinas, [1947], at sacred-texts.com
We next consider God's immutability, and His eternity following on His immutability. On the immutability of God there are two points of inquiry:
(1) Whether God is altogether immutable?
(2) Whether to be immutable belongs to God alone?
Objection 1: It seems that God is not altogether immutable. For whatever moves itself is in some way mutable. But, as Augustine says (Gen. ad lit viii, 20), "The Creator Spirit moves Himself neither by time, nor by place." Therefore God is in some way mutable.
Objection 2: Further, it is said of Wisdom, that "it is more mobile than all things active [Vulg.'mobilior']" (Wis. 7:24). But God is wisdom itself; therefore God is movable.
Objection 3: Further, to approach and to recede signify movement. But these are said of God in Scripture, "Draw nigh to God and He will draw nigh to you" (James 4:8). Therefore God is mutable.
On the contrary, It is written, "I am the Lord, and I change not" (Malachi 3:6).
I answer that, From what precedes, it is shown that God is altogether immutable. First, because it was shown above that there is some first being, whom we call God; and that this first being must be pure act, without the admixture of any potentiality, for the reason that, absolutely, potentiality is posterior to act. Now everything which is in any way changed, is in some way in potentiality. Hence it is evident that it is impossible for God to be in any way changeable. Secondly, because everything which is moved, remains as it was in part, and passes away in part; as what is moved from whiteness to blackness, remains the same as to substance; thus in everything which is moved, there is some kind of composition to be found. But it has been shown above (Q[3], A[7]) that in God there is no composition, for He is altogether simple. Hence it is manifest that God cannot be moved. Thirdly, because everything which is moved acquires something by its movement, and attains to what it had not attained previously. But since God is infinite, comprehending in Himself all the plenitude of perfection of all being, He cannot acquire anything new, nor extend Himself to anything whereto He was not extended previously. Hence movement in no way belongs to Him. So, some of the ancients, constrained, as it were, by the truth, decided that the first principle was immovable.
Reply to Objection 1: Augustine there speaks in a similar way to Plato, who said that the first mover moves Himself; calling every operation a movement, even as the acts of understanding, and willing, and loving, are called movements. Therefore because God understands and loves Himself, in that respect they said that God moves Himself, not, however, as movement and change belong to a thing existing in potentiality, as we now speak of change and movement.
Reply to Objection 2: Wisdom is called mobile by way of similitude, according as it diffuses its likeness even to the outermost of things; for nothing can exist which does not proceed from the divine wisdom by way of some kind of imitation, as from the first effective and formal principle; as also works of art proceed from the wisdom of the artist. And so in the same way, inasmuch as the similitude of the divine wisdom proceeds in degrees from the highest things, which participate more fully of its likeness, to the lowest things which participate of it in a lesser degree, there is said to be a kind of procession and movement of the divine wisdom to things; as when we say that the sun proceeds to the earth, inasmuch as the ray of light touches the earth. In this way Dionysius (Coel. Hier. i) expounds the matter, that every procession of the divine manifestation comes to us from the movement of the Father of light.
Reply to Objection 3: These things are said of God in Scripture metaphorically. For as the sun is said to enter a house, or to go out, according as its rays reach the house, so God is said to approach to us, or to recede from us, when we receive the influx of His goodness, or decline from Him.
Objection 1: It seems that to be immutable does not belong to God alone. For the Philosopher says (Metaph. ii) that "matter is in everything which is moved." But, according to some, certain created substances, as angels and souls, have not matter. Therefore to be immutable does not belong to God alone.
Objection 2: Further, everything in motion moves to some end. What therefore has already attained its ultimate end, is not in motion. But some creatures have already attained to their ultimate end; as all the blessed in heaven. Therefore some creatures are immovable.
Objection 3: Further, everything which is mutable is variable. But forms are invariable; for it is said (Sex Princip. i) that "form is essence consisting of the simple and invariable." Therefore it does not belong to God alone to be immutable.
On the contrary, Augustine says (De Nat. Boni. i), "God alone is immutable; and whatever things He has made, being from nothing, are mutable."
I answer that, God alone is altogether immutable; whereas every creature is in some way mutable. Be it known therefore that a mutable thing can be called so in two ways: by a power in itself; and by a power possessed by another. For all creatures before they existed, were possible, not by any created power, since no creature is eternal, but by the divine power alone, inasmuch as God could produce them into existence. Thus, as the production of a thing into existence depends on the will of God, so likewise it depends on His will that things should be preserved; for He does not preserve them otherwise than by ever giving them existence; hence if He took away His action from them, all things would be reduced to nothing, as appears from Augustine (Gen. ad lit. iv, 12). Therefore as it was in the Creator's power to produce them before they existed in themselves, so likewise it is in the Creator's power when they exist in themselves to bring them to nothing. In this way therefore, by the power of another---namely, of God---they are mutable, inasmuch as they are producible from nothing by Him, and are by Him reducible from existence to non-existence.
If, however, a thing is called mutable by a power in itself, thus also in some manner every creature is mutable. For every creature has a twofold power, active and passive; and I call that power passive which enables anything to attain its perfection either in being, or in attaining to its end. Now if the mutability of a thing be considered according to its power for being, in that way all creatures are not mutable, but those only in which what is potential in them is consistent with non-being. Hence, in the inferior bodies there is mutability both as regards substantial being, inasmuch as their matter can exist with privation of their substantial form, and also as regards their accidental being, supposing the subject to coexist with privation of accident; as, for example, this subject "man" can exist with "not-whiteness" and can therefore be changed from white to not-white. But supposing the accident to be such as to follow on the essential principles of the subject, then the privation of such an accident cannot coexist with the subject. Hence the subject cannot be changed as regards that kind of accident; as, for example, snow cannot be made black. Now in the celestial bodies matter is not consistent with privation of form, because the form perfects the whole potentiality of the matter; therefore these bodies are not mutable as to substantial being, but only as to locality, because the subject is consistent with privation of this or that place. On the other hand incorporeal substances, being subsistent forms which, although with respect to their own existence are as potentiality to act, are not consistent with the privation of this act; forasmuch as existence is consequent upon form, and nothing corrupts except it lose its form. Hence in the form itself there is no power to non-existence; and so these kinds of substances are immutable and invariable as regards their existence. Wherefore Dionysius says (Div. Nom. iv) that "intellectual created substances are pure from generation and from every variation, as also are incorporeal and immaterial substances." Still, there remains in them a twofold mutability: one as regards their potentiality to their end; and in that way there is in them a mutability according to choice from good to evil, as Damascene says (De Fide ii, 3,4); the other as regards place, inasmuch as by their finite power they attain to certain fresh places---which cannot be said of God, who by His infinity fills all places, as was shown above (Q[8], A[2]).
Thus in every creature there is a potentiality to change either as regards substantial being as in the case of things corruptible; or as regards locality only, as in the case of the celestial bodies; or as regards the order to their end, and the application of their powers to divers objects, as in the case with the angels; and universally all creatures generally are mutable by the power of the Creator, in Whose power is their existence and non-existence. Hence since God is in none of these ways mutable, it belongs to Him alone to be altogether immutable.
Reply to Objection 1: This objection proceeds from mutability as regards substantial or accidental being; for philosophers treated of such movement.
Reply to Objection 2: The good angels, besides their natural endowment of immutability of being, have also immutability of election by divine power; nevertheless there remains in them mutability as regards place.
Reply to Objection 3: Forms are called invariable, forasmuch as they cannot be subjects of variation; but they are subject to variation because by them their subject is variable. Hence it is clear that they vary in so far as they are; for they are not called beings as though they were the subject of being, but because through them something has being.