Thomas Aquinas, Compendium of Theology

This makes it clear that, in creating, God has no need of pre-existing matter from which to fashion things. No agent needs, prior to his action, what he produces by his action; he needs only what he is unable to produce by his action. The builder requires stones and lumber before he can set to work, because he is unable to produce these materials by his action. On the other hand, he does not presuppose a house, but produces it by his activity. But matter must be produced by God's action since, as has just been proved, everything that exists in any way at all has God as the cause of its existence. Therefore the conclusion follows that God does not presuppose matter in His activity.

Besides, act naturally precedes potency, and hence the notion of principle primarily befits act. But any principle that in creating would presuppose some other principle, would verify the concept of principle only in a secondary way. Accordingly, since God is the principle of things as the first act, whereas matter is a principle as a being in potency, it is unthinkable that matter should be presupposed before God can act.

Furthermore, the more universal a cause is, the more universal its effect is. Particular causes make use of the effects of universal causes for something determinate; and such determination is related to a universal effect as act to potency. Hence any cause that causes something to be in act, utilizing pre-existing material that is in potency to that act, is a particular cause with respect to some more universal cause. But this sort of procedure cannot pertain to God, since He is the first cause, as we showed above. Consequently God does not need matter as a prerequisite to His action. Therefore He has the power to bring things into existence from nothing or, in other words, to create. This is why the Catholic faith professes that He is the Creator.