Thomas Aquinas, Compendium of Theology

The truth set forth in the preceding chapter also discloses the fact that God has brought things into existence not through any necessity of His nature but by His will. A single natural agent produces immediately but one effect, whereas a voluntary agent can produce a variety of effects. The reason for this is that every agent acts in virtue of its form. The natural form, whereby a cause operates naturally, is limited to one for each agent. But intellectual forms, whereby an agent operates through his will, are many. Therefore, since many things are immediately produced by God, as we have just shown, God evidently produces things by His will, and not under the impulse of natural necessity.

Besides, in the order of causes, an agent operating through intellect and will is prior to an agent operating by the necessity of its nature. For an agent operating through his will predetermines for himself the end for the sake of which he acts, whereas a natural cause operates on account of an end predetermined for it by another. But, as is clear from all that has gone before, God is the first agent. Hence He acts through His will, and not by a necessity of His nature.

Moreover, we demonstrated above that God is infinite in power. Consequently He is not determined to this or that effect, but is undetermined with regard to all effects. But what is undetermined regarding various effects, is determined to produce one of them by desire or by the determination of the will. Thus a man who is free to walk or not to walk, walks when he wills. Hence effects proceed from God according to the determination of His will. And so He acts, not by a necessity of His nature, but by His will. This is why the Catholic faith calls the omnipotent God not only "Creator," but also "Maker." For making is properly the action of an artificer who operates by his will. And since every voluntary agent acts in virtue of the conception of his intellect, which is called his word, as we indicated above, and since the Word of God is His Son, the Catholic faith professes that "all things were made" by the Son.