Thomas Aquinas, Compendium of Theology

Second causes do not act except through the power of the first cause; thus instruments operate under the direction of art. Consequently all the agents through which God carries out the order of His government, can act only through the power of God Himself. The action of any of them is caused by God, just as the movement of a mobile object is caused by the motion of the mover. In such event the mover and the movement must be simultaneous. Hence God must be inwardly present to any agent as acting therein whenever He moves the agent to act.

Another point: not only the action of secondary agents but their very existence is caused by God, as was shown above. However, we are not to suppose that the existence of things is caused by God in the same way as the existence of a house is caused by its builder. When the builder departs, the house still remains standing. For the builder causes the existence of the house only in the sense that he works for the existence of the house as a house. Such activity is, indeed, the constructing of the house, and thus the builder is directly the cause of the becoming of the house, a process that ceases when he desists from his labors. But God is directly, by Himself, the cause of very existence, and communicates existence to all things just as the sun communicates light to the air and to whatever else is illuminated by the sun. The continuous shining of the sun is required for the preservation of light in the air; similarly God must unceasingly confer existence on things if they are to persevere in existence. Thus all things are related to God as an object made is to its maker, and this not only so far as they begin to exist, but so far as they continue to exist. But a maker and the object made must be simultaneous, just as in the case of a mover and the object moved. Hence God is necessarily present to all things to the extent that they have existence. But existence is that which is the most intimately present in all things. Therefore God must be in all things.

Moreover, whoever, through the agency of intermediate causes, carries out the order he has foreseen, must know and arrange the effects of these intermediate causes. Otherwise the effects would occur outside the order he has foreseen. The prearranged plan of a governor is more perfect in proportion as his knowledge and design descend to details. For if any detail escapes the advertence of the governor, the disposition of that detail will elude his foresight. We showed above that all things are necessarily subject to divine providence; and divine providence must evidently be most perfect, because whatever is predicated of God must befit Him in the highest possible degree. Consequently the ordinations of His providence must extend to the most minute effects.