(CG 3/65). That God preserves things in being (Book 3. God, the End of Creatures) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

NOTHING gives being except in so much as it is an actual being. But God preserves things in actuality.

5. The order of effects is according to the order of causes. But among all effects the first is being: all other things, as they proceed from their cause, are determinations of being. Therefore being is the proper effect of the prime agent, and all other things act inasmuch as they act in the power of the prime agent. Secondary agents, which are in a manner particular determinants of the action of the prime agent, have for the proper effects of their action other perfections determinant of being.

6. What is essentially of a certain nature, is properly the cause of that which comes to have that nature only by participation. But God alone is being by essence, all others are beings by participation. Therefore the being of everything that exists is an effect properly due to God; so that anything that brings anything else into being does so insomuch as it acts in the power of God.

Hence it is said: God created all things to be (Wisd. i, 14).