Objection 1. It seems that whatever God wills He wills necessarily. For everything eternal is necessary. But whatever God wills, He wills from eternity, for otherwise His will would be mutable. Therefore whatever He wills, He wills necessarily.
Objection 2. Further, God wills things apart from Himself, inasmuch as He wills His own goodness. Now God wills His own goodness necessarily. Therefore He wills things apart from Himself necessarily.
Objection 3. Further, whatever belongs to the nature of God is necessary, for God is of Himself necessary being, and the principle of all necessity, as above shown (2, 3). But it belongs to His nature to will whatever He wills; since in God there can be nothing over and above His nature as stated in Metaph. v, 6. Therefore whatever He wills, He wills necessarily.
Objection 4. Further, being that is not necessary, and being that is possible not to be, are one and the same thing. If, therefore, God does not necessarily will a thing that He wills, it is possible for Him not to will it, and therefore possible for Him to will what He does not will. And so the divine will is contingent upon one or the other of two things, and imperfect, since everything contingent is imperfect and mutable.
Objection 5. Further, on the part of that which is indifferent to one or the other of two things, no action results unless it is inclined to one or the other by some other power, as the Commentator [Averroes] says in Phys. ii. If, then, the Will of God is indifferent with regard to anything, it follows that His determination to act comes from another; and thus He has some cause prior to Himself.
Objection 6. Further, whatever God knows, He knows necessarily. But as the divine knowledge is His essence, so is the divine will. Therefore whatever God wills, He wills necessarily.
On the contrary, The Apostle says (Ephesians 1:11): "Who worketh all things according to the counsel of His will." Now, what we work according to the counsel of the will, we do not will necessarily. Therefore God does not will necessarily whatever He wills.
I answer that, There are two ways in which a thing is said to be necessary, namely, absolutely, and by supposition. We judge a thing to be absolutely necessary from the relation of the terms, as when the predicate forms part of the definition of the subject: thus it is absolutely necessary that man is an animal. It is the same when the subject forms part of the notion of the predicate; thus it is absolutely necessary that a number must be odd or even. In this way it is not necessary that Socrates sits: wherefore it is not necessary absolutely, though it may be so by supposition; for, granted that he is sitting, he must necessarily sit, as long as he is sitting. Accordingly as to things willed by God, we must observe that He wills something of absolute necessity: but this is not true of all that He wills. For the divine will has a necessary relation to the divine goodness, since that is its proper object. Hence God wills His own goodness necessarily, even as we will our own happiness necessarily, and as any other faculty has necessary relation to its proper and principal object, for instance the sight to color, since it tends to it by its own nature. But God wills things apart from Himself in so far as they are ordered to His own goodness as their end. Now in willing an end we do not necessarily will things that conduce to it, unless they are such that the end cannot be attained without them; as, we will to take food to preserve life, or to take ship in order to cross the sea. But we do not necessarily will things without which the end is attainable, such as a horse for a journey which we can take on foot, for we can make the journey without one. The same applies to other means. Hence, since the goodness of God is perfect, and can exist without other things inasmuch as no perfection can accrue to Him from them, it follows that His willing things apart from Himself is not absolutely necessary. Yet it can be necessary by supposition, for supposing that He wills a thing, then He is unable not to will it, as His will cannot change.
Reply to Objection 1. From the fact that God wills from eternity whatever He wills, it does not follow that He wills it necessarily; except by supposition.
Reply to Objection 2. Although God necessarily wills His own goodness, He does not necessarily will things willed on account of His goodness; for it can exist without other things.
Reply to Objection 3. It is not natural to God to will any of those other things that He does not will necessarily; and yet it is not unnatural or contrary to His nature, but voluntary.
Reply to Objection 4. Sometimes a necessary cause has a non-necessary relation to an effect; owing to a deficiency in the effect, and not in the cause. Even so, the sun's power has a non-necessary relation to some contingent events on this earth, owing to a defect not in the solar power, but in the effect that proceeds not necessarily from the cause. In the same way, that God does not necessarily will some of the things that He wills, does not result from defect in the divine will, but from a defect belonging to the nature of the thing willed, namely, that the perfect goodness of God can be without it; and such defect accompanies all created good.
Reply to Objection 5. A naturally contingent cause must be determined to act by some external power. The divine will, which by its nature is necessary, determines itself to will things to which it has no necessary relation.
Reply to Objection 6. As the divine essence is necessary of itself, so is the divine will and the divine knowledge; but the divine knowledge has a necessary relation to the thing known; not the divine will to the thing willed. The reason for this is that knowledge is of things as they exist in the knower; but the will is directed to things as they exist in themselves. Since then all other things have necessary existence inasmuch as they exist in God; but no absolute necessity so as to be necessary in themselves, in so far as they exist in themselves; it follows that God knows necessarily whatever He wills, but does not will necessarily whatever He wills.
Source: Summa Theologica (New Advent)