Objection 1. It seems that God is not in all things. For what is above all things is not in all things. But God is above all, according to the Psalm (Psalm 112:4), "The Lord is high above all nations," etc. Therefore God is not in all things.
Objection 2. Further, what is in anything is thereby contained. Now God is not contained by things, but rather does He contain them. Therefore God is not in things but things are rather in Him. Hence Augustine says (Octog. Tri. Quaest. qu. 20), that "in Him things are, rather than He is in any place."
Objection 3. Further, the more powerful an agent is, the more extended is its action. But God is the most powerful of all agents. Therefore His action can extend to things which are far removed from Him; nor is it necessary that He should be in all things.
Objection 4. Further, the demons are beings. But God is not in the demons; for there is no fellowship between light and darkness (2 Corinthians 6:14). Therefore God is not in all things.
On the contrary, A thing is wherever it operates. But God operates in all things, according to Isaiah 26:12, "Lord... Thou hast wrought all our works in [Vulgate: 'for'] us." Therefore God is in all things.
I answer that, God is in all things; not, indeed, as part of their essence, nor as an accident, but as an agent is present to that upon which it works. For an agent must be joined to that wherein it acts immediately and touch it by its power; hence it is proved in Phys. vii that the thing moved and the mover must be joined together. Now since God is very being by His own essence, created being must be His proper effect; as to ignite is the proper effect of fire. Now God causes this effect in things not only when they first begin to be, but as long as they are preserved in being; as light is caused in the air by the sun as long as the air remains illuminated. Therefore as long as a thing has being, God must be present to it, according to its mode of being. But being is innermost in each thing and most fundamentally inherent in all things since it is formal in respect of everything found in a thing, as was shown above (Question 7, Article 1). Hence it must be that God is in all things, and innermostly.
Reply to Objection 1. God is above all things by the excellence of His nature; nevertheless, He is in all things as the cause of the being of all things; as was shown above in this article.
Reply to Objection 2. Although corporeal things are said to be in another as in that which contains them, nevertheless, spiritual things contain those things in which they are; as the soul contains the body. Hence also God is in things containing them; nevertheless, by a certain similitude to corporeal things, it is said that all things are in God; inasmuch as they are contained by Him.
Reply to Objection 3. No action of an agent, however powerful it may be, acts at a distance, except through a medium. But it belongs to the great power of God that He acts immediately in all things. Hence nothing is distant from Him, as if it could be without God in itself. But things are said to be distant from God by the unlikeness to Him in nature or grace; as also He is above all by the excellence of His own nature.
Reply to Objection 4. In the demons there is their nature which is from God, and also the deformity of sin which is not from Him; therefore, it is not to be absolutely conceded that God is in the demons, except with the addition, "inasmuch as they are beings." But in things not deformed in their nature, we must say absolutely that God is.
Source: Summa Theologica (New Advent)