Thomas Aquinas: Knowing God and Demonstrating His Existence

“The heavens declare the glory of God; the skies proclaim the work of his hands. Day after day they pour forth speech; night after night they reveal knowledge. They have no speech, they use no words; no sound is heard from them. Yet their voice goes out into all the earth, their words to the ends of the world.” (Psalm 19:1–4, NIV).

In this passage, we discover that as we ponder the night sky, we can learn something about our world. Namely that God exists and that he created everything. Today, we’re going to use this passage as a launching point to discuss our task to both define and defend having faith in God.

But before we get to that, I want you to consider this scenario. A young man attends church all his life. He reads the Bible occasionally and attends youth group while he is in high school. He learns that Jesus is the only answer to the questions in his life. He knows that he must have faith in Christ for his salvation. His parents arrange for him to spend time with other Christians so that he doesn’t fall in with the wrong crowd. He appears to follow God, pray, read the Bible, and live a growing Christian faith. From this perspective, everything looks good.

He graduates high school and heads off to college. He joins a Christian club at his college, and they decide to go out on the streets and hand out invitations to the next church service. An atheist begins to speak with the young man and his friends and begins to gently engage them about their beliefs. Their initial assurance begins to fade, however, as the atheist asks carefully worded questions about the things, they think they know “by faith.” The atheist gives no evidence for atheism. He gives no evidence against Christianity either. Instead, he asks questions about “faith” as a way of knowing. He remains calm and friendly as the discussion closes, but by that time one of the Christians is visibly uncomfortable and walks away with doubts clouding his mind.

This scenario isn’t a piece of fiction, unfortunately. This is a real-world scenario that you can go onto YouTube today and watch unfold before your eyes. A man named Peter Boghossian has authored a book called “A Manual for Creating Atheists” and encourages this tactic which he calls “Street Epistemology” as a play on Christians who do street evangelism. Epistemology is the philosophical study of how you come to know and believe things. With this technique, he attacks how someone has “faith” as a way of knowing what they believe. For instance, when someone claims the existence of God, the street epistemologist might word his questions in such a way to suggest that you make this claim based on blind faith. Instead of criticizing the claims of Christianity, he assaults faith by changing the Biblical meaning of faith. Faith in the Bible is centered on trust in God, but he changes the definition so that faith is seen as irrational. In this practice, the atheist suggests that faith is blind faith, and he intends to leave the person questioning their beliefs because they are unaware and unprepared to answer the person.

How do we begin to respond to these things? Daniel Akin, president of Southeastern Baptist Seminary states: “Christians in the 21st century, as never before, must know what they believe and why they believe. They must be able to define and defend the faith” Apologetics is the theological study of the claims of Christianity and the practical application of defending those claims. Did you know that every one of us is an apologist, or at least, we should be? If you have ever had someone ask you, “why do you believe that?”, then the answer you gave is an apologetic. This doesn’t mean we are to apologize for our faith, but instead, to stand firm on what we believe and defend it. We get the word and the command to practice apologetics from 1 Peter 3:15, which says:

“But in your hearts revere Christ as Lord. Always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have. But do this with gentleness and respect” (1 Peter 3:15, NIV)

Today, I want us to consider two important questions related to the task of apologetics and, specifically, to the atheist who charges that Christian belief is irrationally based on blind faith alone. The first question to ask is whether we can know that God exists and if it is reasonable to believe this; and the second is, can we demonstrate the existence of God by thinking through the evidence of cause and effect in the world.

Can We Have Knowledge of God?

The first thing I want to explore is whether or not we can know that God exists. In other words, is the existence of God known to us, and is there evidence that demonstrates that knowledge. The great theologian Thomas Aquinas can help us understand this better. In his theological work, he states:

To know that God exists in a general and confused way is implanted in us by nature, inasmuch as God is man’s beatitude. For man naturally desires happiness, and what is naturally desired by man must be naturally know to him. (Kreeft, Shorter Summa, 49.)

What Thomas is saying here is that there is a certain way that we can know God because God in and of himself has planted some desire for himself in us. C.S. Lewis echoes this stating, “Creatures are not born with desires unless satisfaction for these desires exists. A baby feels hunger; well, there is such a thing as food. A duckling wants to swim; well, there is such a thing as water. Men feel sexual desire; well, there is such a thing as sex. If I find in myself a desire which no experience in this world can satisfy, the most probable explanation is that I was made for another world” (Lewis, Mere Christianity, 136-137).

One of the most interesting things that some atheists insist upon is the idea that man is naturally atheistic. This is demonstrably false. The vast majority of human beings throughout history have believed in God in some way (Kreeft, 46, see note 2). It is the project of secularism to teach people to become atheistic, but man’s default is to seek God.

The Bible confirms this view in Romans 1 where Paul states that “since the creation of the world God’s invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made, so that people are without excuse” (Romans 1:20). Thomas communicates to us that we have some knowledge of God that is planted in us, but it is not complete knowledge. Aquinas states that we cannot know God and his attributes explicitly from this desire. He contends that we must examine the evidence that God provides through revelation to come to certain knowledge (Kreeft, 49). What he is teaching us (and what Paul confirms in Romans) is that God can be known and demonstrated through his effects, that is, through the things that he has made.

Hebrews 11:1 is often cited as an objection to the knowledge of God and demonstration of God’s existence by our misunderstanding friends, who might claim that to have faith in God means that you merely hope that God exists. Here, the author of Hebrews states, “Now faith is confidence in what we hope for and assurance about what we do not see.” But if you read the text carefully, then you will notice that this is not a definition of faith but defines what living by faith looks like. The author of Hebrews goes on to cite historical examples. Thomas Aquinas also anticipated this objection and responds that “the existence of God and other like truths about God, which can be known by natural reason, are not articles of faith, but are preambles to the articles; for faith presupposes natural knowledge.” (Kreeft, 52). Thomas is affirming that Hebrews 11 isn’t a treatise on the knowledge of God through faith.

We can know God by his effects, by what he has created. Psalm 19 states that the heavens declare the glory of God. Thus, we can seek God, see what he has revealed, and can have faith in his work and promises. Further, we can demonstrate God’s existence to our atheist friends by pointing to his creation as evidence that God is there, and he is not silent.

Can We Demonstrate the Existence of God?

We can even demonstrate through creation that it is reasonable to believe in God’s existence, as we just discussed, and thereby refute the atheist who charges that we believe in God blindly. This is a function of defensive apologetics. But we are not called just to defend our faith but to proclaim it. We must flip the script and become missional ourselves. One way we can do that using an apologetic approach is to demonstrate how God necessarily exists because of evidence found in the principle of cause and effect. Again, Thomas Aquinas can help us here. He states,

In the world of sense, we find there is an order of efficient causes. There is no case known in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible. Now in efficient causes, it is not possible to go on to infinity…[and] to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect…Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God. (Kreeft, 60).

Here Aquinas appeals to the real nature of cause and effect in our world. Nothing can be the cause of itself, that’s like putting the cart before the horse- it doesn’t make much sense to speculate on. So, we see in reality a chain of cause and effect. This can be in time, like how one event leads to another such as how parents are born before their children, or this can be logical cause and effect such as the stories of a building wherein the ground floor is causally prior to the second floor. (Kreeft, 60, note 21). Thomas demonstrates that it is impossible to suggest that the chain of causes in reality is regressively infinite- meaning that there is no first cause. This is similar to suggesting that someone climb an infinite set of stairs steps that doesn’t have a first step- how does a person take the first step if there is no first step? It is logically necessary that a first step exists for there to be an effect of a set of stairs (even if those stairs extend infinitely forward from the point of the first step). In the same way, there must be an efficient first cause to all of the effects we witness in the world, and this is found in God.

Even the popular astronomer Carl Sagan, when explaining how stars contain the necessary materials for life, once said, “If you wish to make an apple pie from scratch, you must first invent the universe.” (Carl Sagan, Cosmos, 230). He made this statement implying that the only thing made “from scratch” or out of nothing is the universe itself, therefore, everything else in existence is contingent on the universe’s beginning. To describe the nature of all matter, Sagan (an atheist) appealed to a first cause. Even atheists are unable to escape the necessity of a first cause, and we can use this evidence to demonstrate the existence of God positively to help us begin gospel conversations.

Not only can we know that God exists through the things that are created, but we can also demonstrate that God exists through reasoning from the effects that we see in creation to the necessity of a first cause. This is an apologetic task, and we are commanded to always be ready to provide an answer. Thus, we must pick up the mantle of the apologist and carefully define why we believe what we believe along with challenging others to have the same faith as us.


Kreeft, Peter. A Shorter Summa: The Essential Philosophical Passages of Saint Thomas Aquinas’ Summa Theologica. First edition. San Francisco: Ignatius Press, 1993.

Lewis, C. S., and Kathleen Norris. Mere Christianity. Revised & Enlarged edition. San Francisco: HarperOne, 2015.

Sagan, Carl. Cosmos. 1st edition. New York: Random House, 1980.

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