In Sum. Cont. Gent. 2.15, Aquinas explains how it is that “God is the Cause of All Being.” This fundamental truth is affirmed in passages like Genesis 1, Acts 17:28, and Romans 11:36. I have summarized Aquinas’ argument below. So we can appreciate what it means for God to be the Cause of All Being.

1. Every Mystery Has a Beginning

Aquinas begins with the simple fact that all things have an origin. A book must have an author; a house must have a builder. In the same way, every being that exists must receive existence from something else. But this cannot regress without end. There must be a first source of being, uncaused and independent. Aquinas identifies this source as God, echoing Genesis 1:1: “In the beginning God created the heavens and the earth.”

2. There Can Only Be One “First”

Aquinas shows that there cannot be two ultimate causes of being, just as there cannot be two “firsts.” If there were two, neither would be truly first. In Scripture, God declares, “I am the first and I am the last; besides me there is no god” (Isa 44:6). Only God is the one who simply is—the “I AM” of Exodus 3:14.

3. God Is the Most Complete Being

Created things are incomplete and dependent. Stars shine only because they are fueled; trees grow only because they receive sunlight and water. God alone is being in himself. As Psalm 36:9 proclaims, “With you is the fountain of life; in your light we see light.”

4. Like Ruler, Like Kingdom

Just as a king orders and sustains his kingdom, so God gives being to all creatures. The apostle Paul makes the same point: “From him and through him and to him are all things” (Rom 11:36).

5. God Doesn’t Just Have Life—He Is Life

Aquinas insists that God does not merely possess existence. He is existence: “God himself is his own being” (SCG 2.15). Every creature has life as a received gift, but God is life itself. Christ declares, “I am the way, and the truth, and the life” (John 14:6). Our existence is like branches drawing life from the vine (John 15:5).

6. Everything Needs Something

Every object around us depends on something else for its existence: the chair on wood, the wood on a tree, the tree on soil and sunlight. But this chain of dependence must end in one who is not dependent. Aquinas argues, “All beings must be from one first being, which is being itself” (SCG 2.15). Scripture agrees: “Every house is built by someone, but the builder of all things is God” (Heb 3:4).

7. Perfect Plans Make Perfect Things

Because God is pure being and perfect, he holds within himself the plan and possibility of all that exists. Aquinas says, “That which is being through its essence must be the cause of being to all” (SCG 2.15). Just as a master craftsman fashions works out of his wisdom, so the Creator brings forth creation out of his perfect goodness. Psalm 104:24 declares, “O Lord, how manifold are your works! In wisdom you have made them all.”

Conclusion
Aquinas’ reasoning in Summa contra Gentiles 2.15 shows why Christian theology confesses God as the source and sustainer of all that is. He is not one being among others but Being itself. All creatures depend on him, while he depends on nothing. This is the meaning of Paul’s words: “For from him and through him and to him are all things. To him be glory forever. Amen” (Rom 11:36).

What Does God’s Word Tell Us?

The Bible confirms what Aquinas discovered through careful thinking:

Psalm 95:5 declares, “The sea is his, for he made it, and his hands formed the dry land.” God shaped both the vast oceans and the solid ground beneath our feet.

John 1:3 reveals an amazing truth: “All things were made through him, and without him was not any thing made that was made.” Everything comes from God. Nothing exists without him.

Romans 11:36 shows us the complete picture: “For from him and through him and to him are all things. To him be glory forever.” God is the beginning, the middle, and the end of everything that exists.

Why Should We Care?

Aquinas helps us see that God isn’t just somewhere “out there” in heaven, separate from our daily lives. He is the reason your heart beats, the reason flowers bloom, the reason stars shine. Every good thing you experience comes from him.

When you see a sunset, remember that God is the source of both the sun and your ability to see its beauty. When you hug someone you love, remember that God is the source of both that person’s existence and your capacity to love them.

This understanding can fill us with wonder and gratitude. We’re not accidents floating in empty space. We’re beloved creations of the God who is the source of all existence, the one who keeps everything going by his power and love.

The next time you look around at the world, you can smile and think: “All of this points back to God, the amazing source of everything that is.”

Here is Aquinas’ work:

Chapter XV

that god is to all things the cause of being

Now, since we have proved that God is the source of being to some things, we must further show that everything besides Himself is from Him.

For whatever belongs to a thing otherwise than as such, belongs to it through some cause, as white to a man: because that which has no cause is something first and immediate, wherefore it must needs belong to the thing essentially and as such. Now it is impossible for any one thing to belong to two and to both of them as such. For that which is said of a thing as such, does not go beyond that thing: for instance to have three angles equal to two right angles does not go beyond a triangle. Accordingly if something belongs to two things, it will not belong to both as such: wherefore it is impossible for any one thing to be predicated of two so as to be said of neither by reason of a cause, but it is necessary that either the one be the cause of the other,—for instance fire is the cause of heat in a mixed body, and yet each is called hot;—or else some third thing must be cause of both, for instance fire is the cause of two candles giving light. Now being is said of everything that is. Wherefore it is impossible that there be two things neither of which has a cause of its being, but either both the things in question must have their being through a cause, or else the one must be the cause of being to the other. Hence everything that, in any way whatever, is, must needs be from that to which nothing is a cause of being. Now we have proved above that God is this being to which nothing is a cause of its being. Therefore from Him is everything that, in any way whatever, is. If however it be said that being is not a univocal predicate, the above conclusion follows none the less. For it is not said of many equivocally, but analogically: and thus it is necessary to be brought back to one thing.

Moreover. That which belongs to a thing by its nature, and not by some other cause, cannot be diminished and deficient therein. For if something essential be subtracted from or added to a nature, there will be at once another nature: even as it happens in numbers, where the addition or subtraction of unity changes the species. And if the nature or quiddity of a thing remain entire, although something is found to be diminished, it is clear that this does not depend simply on that nature, but on something else, through the absence of which it is diminished. Wherefore that which belongs to one thing less than to others, belongs to it not through its nature alone, but through some other cause. Consequently that thing will be the cause of all in a certain genus, to which thing the predication of that genus belongs above all; hence that which is most hot is seen to be the cause of heat in all things hot, and that which is most light is the cause of all things that have light. Now God is being above all, as we have proved in the First Book.² Therefore He is the cause of all of which being is predicated.

Further. The order of causes must needs correspond to the order of effects, since effects are proportionate to their causes. Wherefore, as proper effects are reduced to their proper causes, so that which is common in proper effects must needs be reduced to some common cause: even so, above the particular causes of the generation of this or that thing, is the sun the universal cause of generation; and the king is the universal cause of government in his kingdom, above the wardens of the kingdom and of each city. Now being is common to all. Therefore above all causes there must be a cause to which it belongs to give being. But God is the first cause, as shown above. Therefore it follows that all things that are, are from God.

Again. That which is said to be essentially so and so is the cause of all that are so by participation: thus fire is the cause of all things ignited as such. Now God is being by His essence, because He is being itself: whereas everything else is being by participation: for there can be but one being that is its own being, as was proved in the First Book.³ Therefore God is the cause of being to all other things.

Further. Everything that is possible to be and not to be has a cause: because considered in itself it is indifferent to either, so that there must needs be something else that determines it to one. Wherefore, since we cannot proceed to infinity, there must needs be some necessary thing that is the cause of all things that it is possible to be and not to be. Now there is a necessary thing that has a cause of its necessity: and here again we cannot proceed to infinity, so that we must come to something that is of itself necessary to be. And this can be but one, as we showed in the First Book: and this is God. Therefore everything other than Him must be reduced to Him as the cause of its being.

Moreover. God is the maker of a thing, inasmuch as He is in act, as we have proved above. Now by His actuality and perfection He contains all the perfections of things, as we have shown in the First Book;⁶ and thus He is virtually all things. Therefore He is the maker of all. But this would not be if something else were of a nature to be otherwise than from Him: for nothing is of a nature to be from another, and not to be from another, since if it be of a nature not to be from another, it is of itself necessary to be, and thus can never be from another. Therefore nothing can be except from God.

Again. The imperfect originate from the perfect, as seed from an animal. Now God is the most perfect being and the sovereign good, as was shown in the First Book. Therefore He is to all things the cause of being, especially since it was proved that there can be but one such thing.

This is confirmed by divine authority. For it is said in the psalm: Who made heaven and earth, the sea, and all the things that are in them: and (Jo. 1:3): All things were made by Him, and without Him was made nothing: and (Rom. 11:36): Of Him, and by Him, and in Him are all things: to Him be glory for ever.

This sets aside the error of the ancient physicists who asserted that certain bodies had no cause of their being: likewise of some who say that God is not the cause of the substance of heaven, but only of its movement.