Objection 1. It seems that the Will of God is changeable. For the Lord says (Genesis 6:7): "It repenteth Me that I have made man." But whoever repents of what he has done, has a changeable will. Therefore God has a changeable will.
Objection 2. Further, it is said in the person of the Lord: "I will speak against a nation and against a kingdom, to root out, and to pull down, and to destroy it; but if that nation shall repent of its evil, I also will repent of the evil that I have thought to do to them" (Jeremiah 18:7-8) Therefore God has a changeable will.
Objection 3. Further, whatever God does, He does voluntarily. But God does not always do the same thing, for at one time He ordered the law to be observed, and at another time forbade it. Therefore He has a changeable will.
Objection 4. Further, God does not will of necessity what He wills, as said before (3). Therefore He can both will and not will the same thing. But whatever can incline to either of two opposites, is changeable substantially; and that which can exist in a place or not in that place, is changeable locally. Therefore God is changeable as regards His will.
On the contrary, It is said: "God is not as a man, that He should lie, nor as the son of man, that He should be changed" (Numbers 23:19).
I answer that, The will of God is entirely unchangeable. On this point we must consider that to change the will is one thing; to will that certain things should be changed is another. It is possible to will a thing to be done now, and its contrary afterwards; and yet for the will to remain permanently the same: whereas the will would be changed, if one should begin to will what before he had not willed; or cease to will what he had willed before. This cannot happen, unless we presuppose change either in the knowledge or in the disposition of the substance of the willer. For since the will regards good, a man may in two ways begin to will a thing. In one way when that thing begins to be good for him, and this does not take place without a change in him. Thus when the cold weather begins, it becomes good to sit by the fire; though it was not so before. In another way when he knows for the first time that a thing is good for him, though he did not know it before; hence we take counsel in order to know what is good for us. Now it has already been shown that both the substance of God and His knowledge are entirely unchangeable (9, 1; 14, 15). Therefore His will must be entirely unchangeable.
Reply to Objection 1. These words of the Lord are to be understood metaphorically, and according to the likeness of our nature. For when we repent, we destroy what we have made; although we may even do so without change of will; as, when a man wills to make a thing, at the same time intending to destroy it later. Therefore God is said to have repented, by way of comparison with our mode of acting, in so far as by the deluge He destroyed from the face of the earth man whom He had made.
Reply to Objection 2. The will of God, as it is the first and universal cause, does not exclude intermediate causes that have power to produce certain effects. Since however all intermediate causes are inferior in power to the first cause, there are many things in the divine power, knowledge and will that are not included in the order of inferior causes. Thus in the case of the raising of Lazarus, one who looked only on inferior causes might have said: "Lazarus will not rise again," but looking at the divine first cause might have said: "Lazarus will rise again." And God wills both: that is, that in the order of the inferior cause a thing shall happen; but that in the order of the higher cause it shall not happen; or He may will conversely. We may say, then, that God sometimes declares that a thing shall happen according as it falls under the order of inferior causes, as of nature, or merit, which yet does not happen as not being in the designs of the divine and higher cause. Thus He foretold to Ezechias: "Take order with thy house, for thou shalt die, and not live" (Isaiah 38:1). Yet this did not take place, since from eternity it was otherwise disposed in the divine knowledge and will, which is unchangeable. Hence Gregory says (Moral. xvi, 5): "The sentence of God changes, but not His counsel"--that is to say, the counsel of His will. When therefore He says, "I also will repent," His words must be understood metaphorically. For men seem to repent, when they do not fulfill what they have threatened.
Reply to Objection 3. It does not follow from this argument that God has a will that changes, but that He sometimes wills that things should change.
Reply to Objection 4. Although God's willing a thing is not by absolute necessity, yet it is necessary by supposition, on account of the unchangeableness of the divine will, as has been said above (Article 3).
Source: Summa Theologica (New Advent)