From the Summa Theologiae, Pt. I:

Q. 8, a. 1: Whether God is in all things?

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 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 Is. 26:12, "Lord . . . Thou hast wrought all our works in [Vulg.: '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 (7, 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 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.

 

Q. 8, a. 2.: Whether God is everywhere?

Objection 1. It seems that God is not everywhere. For to be everywhere means to be in every place. But to be in every place does not belong to God, to Whom it does not belong to be in place at all; for "incorporeal things," as Boethius says (De Hebdom.), "are not in a place." Therefore God is not everywhere.

On the contrary, It is written, "I fill heaven and earth." (Jeremiah 23:24).

I answer that, Since place is a thing, to be in place can be understood in a twofold sense; either by way of other things--i.e. as one thing is said to be in another no matter how; and thus the accidents of a place are in place; or by a way proper to place; and thus things placed are in a place. Now in both these senses, in some way God is in every place; and this is to be everywhere.

First, as He is in all things giving them being, power and operation; so He is in every place as giving it existence and locative power. Again, things placed are in place, inasmuch as they fill place; and God fills every place; not, indeed, like a body, for a body is said to fill place inasmuch as it excludes the co-presence of another body; whereas by God being in a place, others are not thereby excluded from it; indeed, by the very fact that He gives being to the things that fill every place, He Himself fills every place.

Reply to Objection 1. Incorporeal things are in place not by contact of dimensive quantity, as bodies are but by contact of power.

 

Q. 8, a. 3: Whether God is everywhere by essence, presence and power?

Objection 1. It seems that the mode of God's existence in all things is not properly described by way of essence, presence and power. For what is by essence in anything, is in it essentially. But God is not essentially in things; for He does not belong to the essence of anything. Therefore it ought not to be said that God is in things by essence, presence and power.

I answer that, God is said to be in a thing in two ways; in one way after the manner of an efficient cause; and thus He is in all things created by Him; in another way he is in things as the object of operation is in the operator; and this is proper to the operations of the soul, according as the thing known is in the one who knows; and the thing desired in the one desiring. In this second way God is especially in the rational creature which knows and loves Him actually or habitually. And because the rational creature possesses this prerogative by grace, as will be shown later (12). He is said to be thus in the saints by grace.

But how He is in other things created by Him, may be considered from human affairs. A king, for example, is said to be in the whole kingdom by his power, although he is not everywhere present. Again a thing is said to be by its presence in other things which are subject to its inspection; as things in a house are said to be present to anyone, who nevertheless may not be in substance in every part of the house. Lastly, a thing is said to be by way of substance or essence in that place in which its substance may be. Now there were some (the Manichees) who said that spiritual and incorporeal things were subject to the divine power; but that visible and corporeal things were subject to the power of a contrary principle. Therefore against these it is necessary to say that God is in all things by His power.

But others, though they believed that all things were subject to the divine power, still did not allow that divine providence extended to these inferior bodies, and in the person of these it is said, "He walketh about the poles of the heavens; and He doth not consider our things [Vulg.: 'He doth not consider . . . and He walketh,' etc.]" (Job 22:14). Against these it is necessary to say that God is in all things by His presence.

Further, others said that, although all things are subject to God's providence, still all things are not immediately created by God; but that He immediately created the first creatures, and these created the others. Against these it is necessary to say that He is in all things by His essence.

Therefore, God is in all things by His power, inasmuch as all things are subject to His power; He is by His presence in all things, as all things are bare and open to His eyes; He is in all things by His essence, inasmuch as He is present to all as the cause of their being.

Reply to Objection 1. God is said to be in all things by essence, not indeed by the essence of the things themselves, as if He were of their essence; but by His own essence; because His substance is present to all things as the cause of their being.