Objection 1. It seems that God does not will things apart from Himself. For the divine will is the divine existence. But God is not other than Himself. Therefore He does not will things other than Himself.
Objection 2. Further, the willed moves the willer, as the appetible the appetite, as stated in De Anima iii, 54. If, therefore, God wills anything apart from Himself, His will must be moved by another; which is impossible.
Objection 3. Further, if what is willed suffices the willer, he seeks nothing beyond it. But His own goodness suffices God, and completely satisfies His will. Therefore God does not will anything apart from Himself.
Objection 4. Further, acts of will are multiplied in proportion to the number of their objects. If, therefore, God wills Himself and things apart from Himself, it follows that the act of His will is manifold, and consequently His existence, which is His will. But this is impossible. Therefore God does not will things apart from Himself.
On the contrary, The Apostle says (1 Thessalonians 4:3): "This is the will of God, your sanctification."
I answer that, God wills not only Himself, but other things apart from Himself. This is clear from the comparison which we made above (Article 1). For natural things have a natural inclination not only towards their own proper good, to acquire it if not possessed, and, if possessed, to rest therein; but also to spread abroad their own good amongst others, so far as possible. Hence we see that every agent, in so far as it is perfect and in act, produces its like. It pertains, therefore, to the nature of the will to communicate as far as possible to others the good possessed; and especially does this pertain to the divine will, from which all perfection is derived in some kind of likeness. Hence, if natural things, in so far as they are perfect, communicate their good to others, much more does it appertain to the divine will to communicate by likeness its own good to others as much as possible. Thus, then, He wills both Himself to be, and other things to be; but Himself as the end, and other things as ordained to that end; inasmuch as it befits the divine goodness that other things should be partakers therein.
Reply to Objection 1. The divine will is God's own existence essentially, yet they differ in aspect, according to the different ways of understanding them and expressing them, as is clear from what has already been said (13, 4). For when we say that God exists, no relation to any other object is implied, as we do imply when we say that God wills. Therefore, although He is not anything apart from Himself, yet He does will things apart from Himself.
Reply to Objection 2. In things willed for the sake of the end, the whole reason for our being moved is the end, and this it is that moves the will, as most clearly appears in things willed only for the sake of the end. He who wills to take a bitter draught, in doing so wills nothing else than health; and this alone moves his will. It is different with one who takes a draught that is pleasant, which anyone may will to do, not only for the sake of health, but also for its own sake. Hence, although God wills things apart from Himself only for the sake of the end, which is His own goodness, it does not follow that anything else moves His will, except His goodness. So, as He understands things apart from Himself by understanding His own essence, so He wills things apart from Himself by willing His own goodness.
Reply to Objection 3. From the fact that His own goodness suffices the divine will, it does not follow that it wills nothing apart from itself, but rather that it wills nothing except by reason of its goodness. Thus, too, the divine intellect, though its perfection consists in its very knowledge of the divine essence, yet in that essence knows other things.
Reply to Objection 4. As the divine intellect is one, as seeing the many only in the one, in the same way the divine will is one and simple, as willing the many only through the one, that is, through its own goodness.
Source: Summa Theologica (New Advent)