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 [Vulgate 'mobilior']" (Wisdom 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 (Question 3, Article 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.
Source: Summa Theologica (New Advent)