Is God immutable or faithful?

Aristotle (Metaphysics) and Aquinas (Summa Theologica) argue for the existence and attributes of God from the observation of motion or change. Aristotle lists four kinds of motion and change: in substance, in quality, in quantity, and in place. These simple changes do not exhaust the kinds of change – even Aristotle implied there were ten kinds, corresponding to his ten categories (Physics 3.1 at 201a8–9).

More significant kinds of change have to do with interpersonal relations. For example, someone says they will do something and then changes their mind. Or someone makes a promise they are unable to fulfill. People change as they mature, which may include character or personality changes. The meaning of an action may change based on the context.

If we focus on simple changes, as Aristotle and Aquinas did, then their argument concludes that God is immutable, that is, incapable of change as if God were like something immobilized, such as a broken limb immobilized by a cast. Does that express the sense in which God is the same yesterday, today, and forever (Heb. 13:8)? No. The reason is the focus on a narrow range of change.

If we focus on the widest range of change, and God is unable to change in any of these ways, we find that God is always consistent, true to his word, has the same personality, and acts the same way. That is, God is faithful.

Numbers 23:19
God is not a man, that He should lie, Nor a son of man, that He should repent; Has He said, and will He not do it? Or has He spoken, and will He not make it good?

Deuteronomy 32:4
The Rock! His work is perfect, For all His ways are just; A God of faithfulness and without injustice, Righteous and upright is He.

Romans 4:21
being fully assured that what God had promised, He was able also to perform.

1 Thessalonians 5:24
Faithful is He who calls you, and He also will bring it to pass.