How Do We Know God Exists?
Faith or Reason?
Many people assume that believing in God is purely a matter of faith — something you either feel or you do not, and which no argument can touch. Some believers are content with that. Many sceptics are too, because it lets them dismiss belief in God as wishful thinking.
The Catholic Church takes a different view, and a bolder one. She teaches that God’s existence can be known with certainty by human reason, simply by thinking carefully about the world around us (CCC 36). Faith is still needed to know who God is in His inner life — that He is a Trinity, that He became man, that He saves us. But that God exists is something the human mind can reach on its own.
This is not a modern idea. St Paul wrote that God’s “invisible nature, namely, his eternal power and deity, has been clearly perceived in the things that have been made” (Romans 1:20). The First Vatican Council made it formal teaching in 1870.
Where the Arguments Start
The classic arguments for God’s existence do not begin with the Bible, or with religious experience, or with anything a sceptic would reject at the outset. They begin with plain facts about the world that everyone accepts: things change, things are caused, things exist that need not have existed.
The most famous summary is the “Five Ways” of St Thomas Aquinas, written in the thirteenth century. They have been explained, criticised and defended ever since. Here they are in simple terms.
1. From Change
Everything around us is changing. Water heats, leaves fall, children grow. But nothing changes itself from nothing. Water does not heat itself; something heats it. Whatever changes is changed by something else.
That something else may itself be changed by another thing, and so on. But this chain cannot go on without a beginning, any more than a train of carriages can move without an engine. There must be something that causes change without itself being changed — a first, unchanging source of all change. This, says Aquinas, is what everyone calls God.
2. From Causes
The second way is similar but looks at causes rather than change. Everything we see owes its existence to something else. You exist because of your parents, who exist because of theirs. Nothing causes itself, because to do so it would have to exist before it existed.
Again, the chain of causes cannot simply go on with nothing at its start, because then nothing would be causing any of it now. There must be a first cause, which is not itself caused by anything.
3. From Things That Need Not Exist
Look around and you will see that everything in the world is the kind of thing that might not have existed. Mountains, stars, animals, people — all come into being and pass away. None of them has to exist.
But if everything were like that, why is there anything at all? Things that need not exist cannot explain their own existence. Ultimately, there must be something that exists by its very nature — something that cannot not exist, and which gives existence to everything else. This is God.
4. From Degrees of Goodness
We constantly compare things as better or worse, truer or less true, more or less noble. A kind act is better than a cruel one. A wise judgement is better than a foolish one. But we can only measure “more” and “less” against a standard.
Aquinas argues that there must be something that is goodness, truth and being in the fullest possible degree — the source and measure of all lesser goodness. This, too, is God.
5. From Order and Purpose
Finally, the natural world shows order. Things without minds act consistently towards ends: seeds grow into plants, eyes see, the laws of physics hold steady. Things without intelligence do not direct themselves towards goals any more than an arrow aims itself.
Something must direct them — an intelligence that orders all of nature towards its ends. This is what we call God.
Other Pointers to God
The Five Ways are not the only paths. The Catechism also points to the human person (CCC 33).
Conscience. Every person has a sense that some things are truly right and others truly wrong — not just useful or harmful, but good or evil. Where does that moral law come from, and why does it bind us?
The longing for more. Human beings long for perfect truth, goodness, beauty and happiness, and nothing in this world fully satisfies them. As St Augustine put it, our hearts are restless until they rest in God.
Beauty. The beauty of nature, art and music has led countless people to sense a source of beauty beyond the world.
These are not strict proofs in the way the Five Ways aim to be. But they are real signs, and for many people they are the first step.
What the Arguments Do Not Show
It is worth being clear about what these arguments can and cannot do. They show that there is a first cause, a necessary being, a supreme good, an ordering intelligence. They do not show, by themselves, that God is a Trinity, that He became man in Jesus Christ, or that He forgives our sins. For that we need revelation.
They also do not compel belief the way a mathematical proof does. People can always resist an argument, especially one with consequences for how they live. The Church acknowledges that, in practice, many find these questions hard, and that God in His goodness has also revealed Himself directly so that everyone can know Him “with firm certainty and with no admixture of error” (CCC 38).
Why This Matters for Believers
If you already believe in God, you might wonder why any of this matters. It matters for three reasons.
It shows that faith is reasonable. Christian faith is not a leap in the dark. It rests on foundations that reason can examine.
It helps in conversation. Friends, children and colleagues may ask why you believe. Being able to offer reasons, gently and clearly, is part of what St Peter asked of every Christian: “Always be prepared to make a defence to any one who calls you to account for the hope that is in you” (1 Peter 3:15).
It deepens wonder. To realise that every moment of your existence depends on God, the first cause holding all things in being, is to see the world differently. Nothing exists by accident. Everything is held in being by Him.
Going Further
For St Thomas Aquinas’s own treatment, see the Tour of the Summa on the existence of God on our theology site. To continue with what God is like, see What Is God Like?