Summa Theologica

Having previously discussed God (the Trinity) as he exists in himself, Aquinas now examines God as the cause of created things.

God is the creator of all things that exist—including prime matter, or matter in its raw state. We see this from the fact that all beings apart from God do not have being in their own right, but rather participate in being. It is necessary to posit a First Being that is the source of all other being. There must be a single unitary Being that causes the multitude of separate beings that exist. Aquinas’ analogy is that of an iron becoming ignited by fire. The fire (God) bestows its heat on the iron, which receives the fire and thus participates in heat.

Further, God is the final cause of all things—the ultimate reason for their creation. God acts not for his own profit but wholly out of his own goodness; he is the “most perfectly free giver” (241). 

To create is to make something from nothing. God necessarily creates from nothing; otherwise there would have to be something from which he made things, which he did not create. It is impossible for creatures to create in the proper sense of the term.