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 (Question 8, Article 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.
Source: Summa Theologica (New Advent)