Objection 1. It would seem that to be created does not belong to composite and subsisting things. For in the book, De Causis (prop. iv) it is said, "The first of creatures is being." But the being of a thing created is not subsisting. Therefore creation properly speaking does not belong to subsisting and composite things.
Objection 2. Further, whatever is created is from nothing. But composite things are not from nothing, but are the result of their own component parts. Therefore composite things are not created.
Objection 3. Further, what is presupposed in the second emanation is properly produced by the first: as natural generation produces the natural thing, which is presupposed in the operation of art. But the thing supposed in natural generation is matter. Therefore matter, and not the composite, is, properly speaking, that which is created.
On the contrary, It is said (Genesis 1:1): "In the beginning God created heaven and earth." But heaven and earth are subsisting composite things. Therefore creation belongs to them.
I answer that, To be created is, in a manner, to be made, as was shown above (44, 2, ad 2,3). Now, to be made is directed to the being of a thing. Hence to be made and to be created properly belong to whatever being belongs; which, indeed, belongs properly to subsisting things, whether they are simple things, as in the case of separate substances, or composite, as in the case of material substances. For being belongs to that which has being--that is, to what subsists in its own being. But forms and accidents and the like are called beings, not as if they themselves were, but because something is by them; as whiteness is called a being, inasmuch as its subject is white by it. Hence, according to the Philosopher (Metaph. vii, text 2) accident is more properly said to be "of a being" than "a being." Therefore, as accidents and forms and the like non-subsisting things are to be said to co-exist rather than to exist, so they ought to be called rather "concreated" than "created" things; whereas, properly speaking, created things are subsisting beings.
Reply to Objection 1. In the proposition "the first of created things is being," the word "being" does not refer to the subject of creation, but to the proper concept of the object of creation. For a created thing is called created because it is a being, not because it is "this" being, since creation is the emanation of all being from the Universal Being, as was said above (Article 1). We use a similar way of speaking when we say that "the first visible thing is color," although, strictly speaking, the thing colored is what is seen.
Reply to Objection 2. Creation does not mean the building up of a composite thing from pre-existing principles; but it means that the "composite" is created so that it is brought into being at the same time with all its principles.
Reply to Objection 3. This reason does not prove that matter alone is created, but that matter does not exist except by creation; for creation is the production of the whole being, and not only matter.
Source: Summa Theologica (New Advent)