Objection 1. It would seem that God does not work in every agent. For we must not attribute any insufficiency to God. If therefore God works in every agent, He works sufficiently in each one. Hence it would be superfluous for the created agent to work at all.
Objection 2. Further, the same work cannot proceed at the same time from two sources; as neither can one and the same movement belong to two movable things. Therefore if the creature's operation is from God operating in the creature, it cannot at the same time proceed from the creature; and so no creature works at all.
Objection 3. Further, the maker is the cause of the operation of the thing made, as giving it the form whereby it operates. Therefore, if God is the cause of the operation of things made by Him, this would be inasmuch as He gives them the power of operating. But this is in the beginning, when He makes them. Thus it seems that God does not operate any further in the operating creature.
On the contrary, It is written (Isaiah 26:12): "Lord, Thou hast wrought all our works in [Vulgate: 'for'] us."
I answer that, Some have understood God to work in every agent in such a way that no created power has any effect in things, but that God alone is the ultimate cause of everything wrought; for instance, that it is not fire that gives heat, but God in the fire, and so forth. But this is impossible.
First, because the order of cause and effect would be taken away from created things: and this would imply lack of power in the Creator: for it is due to the power of the cause, that it bestows active power on its effect.
Secondly, because the active powers which are seen to exist in things, would be bestowed on things to no purpose, if these wrought nothing through them. Indeed, all things created would seem, in a way, to be purposeless, if they lacked an operation proper to them; since the purpose of everything is its operation. For the less perfect is always for the sake of the more perfect: and consequently as the matter is for the sake of the form, so the form which is the first act, is for the sake of its operation, which is the second act; and thus operation is the end of the creature. We must therefore understand that God works in things in such a manner that things have their proper operation.
In order to make this clear, we must observe that as there are few kinds of causes; matter is not a principle of action, but is the subject that receives the effect of action. On the other hand, the end, the agent, and the form are principles of action, but in a certain order. For the first principle of action is the end which moves the agent; the second is the agent; the third is the form of that which the agent applies to action (although the agent also acts through its own form); as may be clearly seen in things made by art. For the craftsman is moved to action by the end, which is the thing wrought, for instance a chest or a bed; and applies to action the axe which cuts through its being sharp.
Thus then does God work in every worker, according to these three things.
First as an end. For since every operation is for the sake of some good, real or apparent; and nothing is good either really or apparently, except in as far as it participates in a likeness to the Supreme Good, which is God; it follows that God Himself is the cause of every operation as its end.
Again it is to be observed that where there are several agents in order, the second always acts in virtue of the first; for the first agent moves the second to act. And thus all agents act in virtue of God Himself: and therefore He is the cause of action in every agent.
Thirdly, we must observe that God not only moves things to operated, as it were applying their forms and powers to operation, just as the workman applies the axe to cut, who nevertheless at times does not give the axe its form; but He also gives created agents their forms and preserves them in being. Therefore He is the cause of action not only by giving the form which is the principle of action, as the generator is said to be the cause of movement in things heavy and light; but also as preserving the forms and powers of things; just as the sun is said to be the cause of the manifestation of colors, inasmuch as it gives and preserves the light by which colors are made manifest. And since the form of a thing is within the thing, and all the more, as it approaches nearer to the First and Universal Cause; and because in all things God Himself is properly the cause of universal being which is innermost in all things; it follows that in all things God works intimately. For this reason in Holy Scripture the operations of nature are attributed to God as operating in nature, according to Job 10:11: "Thou hast clothed me with skin and flesh: Thou hast put me together with bones and sinews."
Reply to Objection 1. God works sufficiently in things as First Agent, but it does not follow from this that the operation of secondary agents is superfluous.
Reply to Objection 2. One action does not proceed from two agents of the same order. But nothing hinders the same action from proceeding from a primary and a secondary agent.
Reply to Objection 3. God not only gives things their form, but He also preserves them in existence, and applies them to act, and is moreover the end of every action, as above explained.
Source: Summa Theologica (New Advent)