God is present in all things, not as part of their essence, or as an accident, but as the agent is present in what he does, for an agent must be united with, and by his power be joined to, what he does; and hence God, as Universal Cause, is present to all things, not only when they begin to be, but as long as they exist. As light, which is caused in the air by the sun, remains so long as the air is illuminated, so God exists perpetually in all things as their Creator.
God is said to be in every place by His Power, not as a body fills place, excluding every other by the fact of its being there, for, rather, God may be said to fill place, inasmuch as He gives to all things in place that which makes them to be in place. Thus God is present effectively in all created things, but objectively in the rational creature which knows and loves Him by act or habit; in such a manner is God present by grace in the saints.
Thus, therefore. He is present in all things by power, inasmuch as all things are subject to Him; and He is in all things by His Presence, as all things are bare and open to His eyes; and He is in all things by His Essence, inasmuch as He is in all as the Cause of Being, Moreover, that is properly said to be everywhere which is necessarily present in any given place, which is proper to God; for however many places there might be, from their very existence it would follow that God must be in them all, for nothing can exist but by Him. Thus it belongs to God to be everywhere first and of Himself; and it belongs to Him alone that, however many places there might be, in each of them He must be present, not by division of parts, but wholly as He is in Himself.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni