What Is God? A Thomistic Introduction to the Mystery of Divine Being

Part 1: Ask Like Aquinas

When Thomas Aquinas was just five years old, he asked his teacher a question that would shape his entire life: “What is God?”

He didn’t ask, “Does God exist?” or “How do I get to heaven?” He asked something much deeper and much harder. What is God?

And in many ways, the rest of his life—and his towering work, the Summa Theologiae—is an attempt to answer that question. Not just for himself, but for the whole Church.

The Starting Point: We Don’t Know

Aquinas begins his Summa not with divine names or attributes, but with a humble admission: we cannot fully know what God is.

God’s essence—what He is in Himself—is beyond the grasp of our finite minds. No created intellect, not even the angels, can comprehend the divine nature. That may sound like a dead end, but for Aquinas, it is exactly the right place to start. To know God rightly, we have to begin with reverence, not presumption.

So if we cannot know what God is, what can we say?

We can know something about God by knowing what He is not, and by reasoning from His effects in creation. In other words, we approach God indirectly, like someone tracing the light that spills from a hidden source.

Why Definitions Matter

Before we talk about God, we need to define some key terms. Aquinas is methodical. He doesn’t leap into theology without first asking: what does it mean to be a cause? What is a first principle? How do we even form a meaningful sentence about God?

These questions are not distractions. They are foundations. And if we want to build our faith on solid ground, we need to understand what holds it up.

What Is a First Principle?

A first principle is a basic truth that cannot be deduced from anything else. It is the foundation on which everything else rests. In philosophy, first principles are self-evident starting points. In theology, the ultimate first principle is God—the cause of all things, who is not Himself caused by anything.

God is not just a very old being or a very big one. He is the first cause, the source from which everything else flows.

Aquinas and the Four Causes

To understand how something comes to be, Aquinas (following Aristotle) identifies four causes:

1. Material cause – what something is made of

2. Formal cause – its shape or essence

3. Efficient cause – what brings it about

4. Final cause – the purpose for which it exists

Let’s look at two examples.

	A Table	A Statue
Material	Wood	Bronze
Formal	Tableness (design)	Shape of the figure
Efficient	The carpenter	The sculptor
Final	A place for a family meal	To honor or remember someone

These causes help us think about the world in a rich, layered way. They also prepare us to ask: what caused the universe? What is its material? Its form? Its maker? Its purpose?

Eventually, this leads us to ask about God, who is not just one cause among others, but the first cause, the one in whom all causes find their source.

The Mystery of Language

One final challenge: even when we use words like “good,” “wise,” or “powerful” to describe God, we must be careful. Those words mean something when we use them for people. But when we use them for God, they don’t quite work the same way.

Aquinas explains that we can speak about God in three ways:

• Univocally – using a word the same way for God and creatures (not accurate)

• Equivocally – using the same word with totally different meanings (not helpful)

• Analogically – using the word in a way that points to similarity, but also acknowledges difference (just right)

So when we say “God is good,” we don’t mean He is good in the same way we are. We mean that everything good we see in the world must exist in Him in a higher and more perfect way.

Conclusion: Beginning with Wonder

Aquinas doesn’t begin theology with an answer. He begins with a question. And that’s what we’re invited to do as well.

“What is God?” is not a question we can fully answer. But it is the right question to ask. It leads us not into confusion, but into awe. And awe is the beginning of wisdom.

In the next part, we’ll go deeper into the four causes and explore how they help us reason about the world—and about God.