The following is a quote from Aquinas’ master work, Summa Theologica. It speaks of how God must continually uphold the universe for it to continue its existence. (The first sentence is a little dense, but after that he explains it fairly clearly)
Therefore as the becoming of a thing cannot continue when that action of the agent ceases which causes the “becoming” of the effect: so neither can the “being” of a thing continue after that action of the agent has ceased, which is the cause of the effect not only in “becoming” but also in “being.” This is why hot water retains heat after the cessation of the fire’s action; while, on the contrary, the air does not continue to be lit up, even for a moment, when the sun ceases to act upon it, because water is a matter susceptive of the fire’s heat in the same way as it exists in the fire. Wherefore if it were to be reduced to the perfect form of fire, it would retain that form always; whereas if it has the form of fire imperfectly and inchoately, the heat will remain for a time only, by reason of the imperfect participation of the principle of heat. On the other hand, air is not of such a nature as to receive light in the same way as it exists in the sun, which is the principle of light. Therefore, since it has not root in the air, the light ceases with the action of the sun.
“Being” here is “be-ing.” as in sleeping, running, jumping, being. Being is an act. So what this passage is going to conclude is that the universe cannot continue to exist (cannot continue be-ing) without something continually causing its existence, just like light cannot continue unless the source of the light continues to provide the light.
If God were to cease to act on the universe, it would cease to be. This is summarized to us in the Bible, where we are told “…in Him all things consist” or hold together (Colossians 1:17).
