Objection 1. It seems that the will of God is not the cause of things. For Dionysius says (Div. Nom. iv, 1): "As our sun, not by reason nor by pre-election, but by its very being, enlightens all things that can participate in its light, so the divine good by its very essence pours the rays of goodness upon everything that exists." But every voluntary agent acts by reason and pre-election. Therefore God does not act by will; and so His will is not the cause of things.
Objection 2. Further, The first in any order is that which is essentially so, thus in the order of burning things, that comes first which is fire by its essence. But God is the first agent. Therefore He acts by His essence; and that is His nature. He acts then by nature, and not by will. Therefore the divine will is not the cause of things.
Objection 3. Further, Whatever is the cause of anything, through being "such" a thing, is the cause by nature, and not by will. For fire is the cause of heat, as being itself hot; whereas an architect is the cause of a house, because he wills to build it. Now Augustine says (De Doctr. Christ. i, 32), "Because God is good, we exist." Therefore God is the cause of things by His nature, and not by His will.
Objection 4. Further, Of one thing there is one cause. But the created things is the knowledge of God, as said before (14, 8). Therefore the will of God cannot be considered the cause of things.
On the contrary, It is said (Wisdom 11:26), "How could anything endure, if Thou wouldst not?"
I answer that, We must hold that the will of God is the cause of things; and that He acts by the will, and not, as some have supposed, by a necessity of His nature. This can be shown in three ways:
First, from the order itself of active causes. Since both intellect and nature act for an end, as proved in Phys. ii, 49, the natural agent must have the end and the necessary means predetermined for it by some higher intellect; as the end and definite movement is predetermined for the arrow by the archer. Hence the intellectual and voluntary agent must precede the agent that acts by nature. Hence, since God is first in the order of agents, He must act by intellect and will.
This is shown, secondly, from the character of a natural agent, of which the property is to produce one and the same effect; for nature operates in one and the same way unless it be prevented. This is because the nature of the act is according to the nature of the agent; and hence as long as it has that nature, its acts will be in accordance with that nature; for every natural agent has a determinate being. Since, then, the Divine Being is undetermined, and contains in Himself the full perfection of being, it cannot be that He acts by a necessity of His nature, unless He were to cause something undetermined and indefinite in being: and that this is impossible has been already shown (7, 2). He does not, therefore, act by a necessity of His nature, but determined effects proceed from His own infinite perfection according to the determination of His will and intellect.
Thirdly, it is shown by the relation of effects to their cause. For effects proceed from the agent that causes them, in so far as they pre-exist in the agent; since every agent produces its like. Now effects pre-exist in their cause after the mode of the cause. Wherefore since the Divine Being is His own intellect, effects pre-exist in Him after the mode of intellect, and therefore proceed from Him after the same mode. Consequently, they proceed from Him after the mode of will, for His inclination to put in act what His intellect has conceived appertains to the will. Therefore the will of God is the cause of things.
Reply to Objection 1. Dionysius in these words does not intend to exclude election from God absolutely; but only in a certain sense, in so far, that is, as He communicates His goodness not merely to certain things, but to all; and as election implies a certain distinction.
Reply to Objection 2. Because the essence of God is His intellect and will, from the fact of His acting by His essence, it follows that He acts after the mode of intellect and will.
Reply to Objection 3. Good is the object of the will. The words, therefore, "Because God is good, we exist," are true inasmuch as His goodness is the reason of His willing all other things, as said before (2, ad 2).
Reply to Objection 4. Even in us the cause of one and the same effect is knowledge as directing it, whereby the form of the work is conceived, and will as commanding it, since the form as it is in the intellect only is not determined to exist or not to exist in the effect, except by the will. Hence, the speculative intellect has nothing to say to operation. But the power is cause, as executing the effect, since it denotes the immediate principle of operation. But in God all these things are one.
Source: Summa Theologica (New Advent)