What is God? Proving God Exists

Part 4: Proving God Exists

We’ve seen that we cannot know what God is in His essence. We know Him by what He is not, and by what His effects reveal. But now comes a bold claim—we can prove that God exists.

That may sound shocking. In a culture that often treats faith as a blind leap or a private feeling, the idea of a proof for God sounds either arrogant or outdated. But for Aquinas, it is neither.

He believed that reason, rightly used, can show that there must be something beyond the universe. Not just a powerful being, but a necessary, eternal, uncaused cause. In other words—God.

Faith and Reason Together

Let’s be clear: Aquinas never claimed that reason could prove everything about God. The Trinity, the Incarnation, and the full beauty of divine love go beyond what reason can grasp. These are matters of faith.

But the existence of God? Aquinas argues that this much can be known from nature, using reason alone.

In fact, he says that proving God exists is not an act of faith at all, but an act of natural knowledge, based on observing the world and drawing logical conclusions. He calls these things preambles to faith.

Many Proofs, One Truth

Philosophers and theologians throughout history have offered many different arguments for God’s existence, some based on logic, others on moral experience, or beauty, or consciousness. You may have heard of the ontological argument from St. Anselm (which Aquinas himself rejected), or more recent approaches involving fine-tuning or the nature of thought itself.

These are valuable and worth studying. Personally, I have a certain fondness for the ontological argument, even though Aquinas didn’t. But in this series, we’re focusing on Aquinas’s method because it’s rooted in metaphysical realism and begins where most of us begin: with the world we experience.

Aquinas and the Five Ways

In the Summa Theologiae, Aquinas presents five arguments for God’s existence. These are not five totally different ideas, but five ways of looking at the world that all lead to the same conclusion.

Let’s take a brief look at each.

1. The Argument from Motion

We see that things move. But nothing moves itself. Everything that is in motion is moved by something else.

This cannot go on forever. There must be a first mover, something that causes motion without being moved itself.

That unmoved mover is what we call God.

But this can be confusing. In modern English, motion usually means physical movement—like walking across a room or planets orbiting a star. But Aquinas means something broader. For him, motion refers to change of any kind. Change happens when something goes from potency to act. That is, from what it could be to what it actually is. For example: A cold cup of tea is potentially hot. When it is heated, that potential becomes actual. A block of marble is potentially a statue. When the sculptor carves it, that potential becomes real. A child is potentially wise. Through learning and growth, that potential is actualized.

In every case, something must move it from potential to actual. And that “mover” itself must be in act. You can’t give what you don’t have.

So, if there is a chain of things being moved from potential to act, there must be something at the beginning of that chain that has no potential, something that is fully actual. This is what Aquinas calls Pure Act—and that, he says, is God.

2. The Argument from Causation

Every effect has a cause. Nothing causes itself. But there cannot be an infinite chain of causes.

There must be a first cause, one that is not itself caused by anything else.

That first cause is God.

Aquinas isn’t talking about a chronological cause—he’s not saying God came before the universe in time, like a spark before a flame. He’s talking about ontological dependence: the idea that things which exist require something else to explain their existence. Some causes are simultaneous, not sequential. For example, a table causes a pencil not to fall. The table doesn’t come before the pencil in time, but it supports the pencil at every moment. In the same way, creation depends on God not just “once upon a time,” but right now.

3. The Argument from Contingency

Everything we see in the world might not have existed. Trees, rocks, people, stars, they are all contingent. They don’t exist by necessity.

If everything were contingent, then at some point, nothing would have existed. But if nothing existed, nothing could have come into being.

Therefore, there must be at least one necessary being- one that cannot not exist.

That being is God.

We can see this intuitively. Everything around us eventually wears out, breaks down, or dies. Even galaxies dissolve. In physics, this is described by the idea of entropy: systems naturally move toward disorder over time. But if the whole universe is like that—if everything is contingent and runs down—then we must ask: Why is there something at all, rather than nothing? The answer must be a being that is not contingent, that doesn’t break down, that doesn’t depend on anything else. In other words, a necessary being. That is what Aquinas means by God.

4. The Argument from Degrees of Perfection

We recognize that some things are better, truer, or nobler than others.

We say that one person is more generous than another, that one painting is more beautiful, or that one argument is more truthful. These comparisons aren’t just preferences. They imply that there is a scale, some standard by which “more” and “less” make sense.

But if we can say something is more or less good, then there must be something that is most good. Just as fire is hottest and other things are hot by comparison, there must be a maximum—something that possesses the fullness of a quality.

That which is supremely true, good, and noble is what we call God.

This may be hard to grasp in a relativistic culture. People often say, “That’s just your opinion,” or “Everyone has their own truth.” But Aquinas is pointing out something deeper: when we make any meaningful comparison- when we say one thing is better or truer than another- we are implicitly referring to a standard. Just like a crooked line can only be recognized against something straight, every lesser good makes sense only in relation to a greater good. Ultimately, there must be something that is goodness itself, the source and standard of all lesser goods. That is God.

5. The Argument from Design (Teleology)

We observe order in the world. Even things without intelligence- like plants, stars, or atoms- act in regular, purposeful ways.

But how can things act for an end if they do not have minds of their own?

Aquinas draws on Aristotle’s idea of final causality: everything that acts toward a goal must be directed by something with knowledge. Just as an arrow reaches its target only because it is aimed by an archer, unintelligent things in nature reach their ends because they are guided by something greater.

There must be an intelligent designer who directs all things toward their natural ends.

That designer is God.

This is more than an argument from complexity or beauty. It is a metaphysical argument: if things have final causes, there must be something that directs them. This insight also supports the Catholic understanding of predestination—not as fatalism, but as God's wise and loving guidance of creation toward its purpose.

What These Arguments Are—and Are Not

These proofs are not mathematical equations. They are metaphysical demonstrations, grounded in observation and logic.

They do not prove the Christian God in full detail, but they do point to a being who is:

• Unmoved

• Uncaused

• Necessary

• Perfect

• Intelligent

In other words, they point to the kind of God that faith recognizes more fully.

Why This Still Matters

Many modern people assume belief in God is irrational. But Aquinas invites us to consider that belief in God is not just reasonable, it may be required by reason, if we follow it far enough.

This doesn’t replace faith. But it clears the ground. It shows that faith is not blind, and that the mind can truly lead the heart to God.

Coming Next: God and Being

In Part 5, we’ll go deeper. What kind of being is God? What does it mean to say that God does not “exist” the way we do? What is Pure Act? What does it mean that God is Being Itself?

We’ve shown that God must exist. Now let’s ask: What does it mean for God to be?