Short answer
We know God (﷾) exists by reason. Whatever begins to exist has a cause. The universe began to exist, so it has a cause. That cause cannot be made of matter or live inside time, because, on the evidence, both began with the universe. It must have great power. It must also have a will. A cause with no start produced a world with a start date, so someone chose the moment. A force does not choose. In 1929 Edwin Hubble found the galaxies moving apart, and the evidence since points back to a beginning. The Qur'an asks three short questions. Were people made from nothing? Did they make themselves? Did they make the heavens and the earth? None of these can be true, so Someone made them. Islam teaches that this Someone is Allah (﷾), the one God (﷾).
Full explanation
The argument in three steps
The argument has three steps. Step one: whatever begins to exist has a cause. Nobody accepts that a horse or a house appeared from nothing. Step two: the universe began to exist. Al-Ghazali, a Muslim thinker of the eleventh century, argued this from reason alone. Picture the past as a chain of days with no beginning. Such a chain could never have been crossed to arrive at today. Step three: so the universe has a cause. Al-Kindi, a Muslim philosopher of the ninth century, and al-Ghazali gave this argument its classic form. Western philosophers still call it the kalam argument. Kalam is the Arabic name for the Islamic theology that developed it.
Sets out the proof in short steps: whatever comes into being has a cause for its coming into being; the world came into being; so the world has a cause.
In its first discussion, argues against the philosophers' view that the world has no beginning, including the argument that an infinite series of past events is impossible.
Argues from reason that the body of the universe cannot be infinite, and so cannot have existed for an infinite time: the universe had a beginning and was brought into being from nothing by God.
Traces the argument that the universe began and so has a cause to the Muslim theologians of the kalam tradition, above all al-Kindi and al-Ghazali, and defends it in modern form.
The beginning that science found
For most of history, the strongest answer unbelievers had was that the universe had always existed. If it never began, it needs no one to begin it. Physics has taken away that answer's main support, because the evidence now points to a beginning. In 1929 the astronomer Edwin Hubble found that the galaxies are moving apart. So, far enough back, they were together. Eddington, the Cambridge astronomer, wrote that the idea of a beginning was repugnant to him, meaning he found it deeply distasteful. Fred Hoyle, another astronomer, defended a universe with no beginning all his life. He gave the rival theory its name, "Big Bang", on BBC radio in 1949. Today the standard picture of the universe traces it back about 13.8 billion years, to a hot, dense beginning. In 2003 three physicists, Borde, Guth and Vilenkin, showed something more. On the physics we know, a universe that has, on average, been expanding cannot have been expanding for ever. Its past has an edge.
Reports that the farther away a galaxy is, the faster it is moving away from us: the observation that became the evidence for an expanding universe.
Eddington writes that, speaking as a philosopher, he finds the idea of a beginning of the present order of nature repugnant.
Shows that the name "Big Bang" was first used by Fred Hoyle in a BBC radio talk in March 1949. Hoyle was a leading defender of the steady-state universe, which had no beginning.
Shows that a universe which has, on average, been expanding cannot have been expanding for ever: its expanding history must have a boundary in the past.
What the cause must be
So what must this cause be like? It cannot be inside time or made of matter, because both began with the universe. It cannot have a beginning of its own, or we would ask again what caused it. It must have immense power. And it must have a will. Think of a cause that sat for ever with everything in place. It would produce its effect for ever, as water freezes the moment the conditions are met. Yet the universe has a start date. So the moment was chosen, and only someone chooses. The Qur'an puts the whole argument in three questions. Were people made from nothing? Did they make themselves? Did they make the heavens and the earth? All three are impossible. The only answer left is that Someone else made them.
“Were they created of nothing, or were they themselves the creators? Or did they create the heavens and the earth? Nay, they have no firm belief.”
Whatever begins to exist has a cause. Nobody accepts that a horse or a house appeared from nothing.
Did the universe begin?
Yes. A past with no beginning could never have been crossed to arrive at today. And since Hubble found the galaxies moving apart in 1929, the evidence has pointed back to a beginning.
So what follows?
The universe has a cause.
What must the cause be like?
Outside time and not made of matter, because both began with the universe. Without a beginning of its own, or the same question returns. And immensely powerful.
Could it be a blind force?
No. An eternal cause produced a world with a starting date, so the moment was chosen. A force does not choose. Only someone chooses.
The Qur'an asks: were they created from nothing, did they create themselves, or did they create the heavens and the earth? None of these can be true, so Someone made them.
Signs in the heavens and the earth
The Qur'an calls the things of creation ayat, which means signs. It uses the same word for its own verses. It tells the reader to look at these signs and to reason. It also says the heavens and the earth were once joined and then parted. These verses are not a physics textbook, and this page does not claim they describe the Big Bang. They are signs, and they sit easily with what was found thirteen centuries later. The Qur'an always taught that the universe was created. Science never forced that teaching to change.
“On the earth are signs for those of assured Faith, As also in your own selves: Will ye not then see?”
“Do not the Unbelievers see that the heavens and the earth were joined together (as one unit of creation), before we clove them asunder? We made from water every living thing. Will they not then believe?”
Perspectives
Historical
Modern science holds that the universe expanded from a hot, dense state. The evidence points to a beginning. The eternal universe that unbelief relied on has lost its main support.
Hubble found in 1929 that distant galaxies are moving away from us, and faster the farther away they are.
Leading scientists resisted the idea of a beginning. Eddington called it repugnant. Hoyle, who held that the universe had no beginning, named the rival theory the "Big Bang" in 1949.
A universe that has, on average, been expanding cannot have been expanding for ever. Its past has an edge.
The Muslim thinkers al-Kindi and al-Ghazali gave the argument from the beginning of the universe its classic form.
The universe needs no cause
Many atheists answer the argument from a beginning in one of two ways. Some say that at the smallest scale, things happen without causes, so the universe's beginning may have had none. Others say the universe may have had no beginning at all.
Quantum events happen without a cause, so the beginning of the universe could have happened without one too.
Rebuttal
Quantum events happen in a vacuum that has energy and follows laws. They have conditions, even if we cannot predict them. So they are not things coming from nothing.
The universe may have always existed, so it never needed a cause.
Rebuttal
A universe that has, on average, been expanding has an edge in its past. And an endless line of past days could never reach today. The page on whether the universe could have always existed explains this.
Buddhist: the question can be set aside
Buddhism does not teach a Creator. The Buddha set aside questions such as whether the universe is eternal. In the Buddhist texts, the god who thinks he made the world is only the first being reborn after the universe expands.
The Buddha left questions such as whether the universe is eternal unanswered, because they do not help to end suffering.
Rebuttal
Setting a question aside does not answer it. Whether the world has a Maker decides whom we owe thanks and worship, so it bears directly on how to live. The Qur'an asks it plainly: were they created of nothing, or were they themselves the creators?
The god who claims to have made the world was only reborn first. The beings who came after him took him for their creator.
Rebuttal
That story is about a god who is himself born and comes into being. Islam agrees that such a being is not God. The God (﷾) of the Qur'an was never born and has no beginning, and nothing is like Him. The Buddha also taught that there is something unconditioned, which never arises and never changes. So Buddhism itself accepts that not everything is made.
Chinese tradition: keep the customs, leave the question
Many Chinese families keep the customs without asking whether God exists. They visit their ancestors' graves, and pray at temples for wealth or good luck. Popular religion honours many gods, such as the Kitchen God, who serve under the Jade Emperor in the court of Heaven.
What matters is keeping the customs and honouring the ancestors, not settling whether God exists.
Rebuttal
People who pray at a temple for wealth or good fortune hope that someone can hear them and give. That hope already asks the question of God (﷾). Islam answers it: the places of worship are for Allah (﷾) alone, so call on no one beside Him.
Heaven has a court. The Jade Emperor rules over many gods, and the Kitchen God reports each family's behaviour to Heaven every year.
Rebuttal
A court of gods who report to one ruler already points to One above them all. The Qur'an says that if there were other gods besides Allah (﷾), heaven and earth would fall into disorder. And God (﷾) needs no one to report to Him. No eye can take Him in, yet He knows all things.
Confucian: right conduct comes first
Confucius spoke of Heaven but did not talk about spiritual beings, and he gave no proof of God. Many Confucians put right conduct in this life first and leave such questions aside.
Confucius did not talk about spiritual beings; what matters is right conduct in this life.
Rebuttal
Confucius did speak of Heaven. He said Heaven produced the virtue in him, and that one who offends against Heaven has none to whom he can pray. Fung Yu-lan finds that the Heaven Confucius speaks of seems to be a ruling Heaven. Virtue that Heaven gives and wrong that offends Heaven point to One who knows and judges.
Heaven does not speak; it shows itself only in the seasons and in the growth of all things.
Rebuttal
That is where the Qur'an begins too. The night and the day, the sun and the moon, are among His signs. Then the Qur'an says what the signs point to: do not bow to the sun and the moon, but to Allah (﷾), who created them.
Daoist: the Tao, not a Creator
The Tao Te Ching speaks of the Tao, which came before Heaven and Earth and from which all things come. It has no name that can truly be spoken, and it makes no claim on what it produces. Many Daoists see no need for a personal Creator.
Before Heaven and Earth there was the Tao, standing alone and unchanging, and from it all things came.
Rebuttal
Islam agrees that one source came before heaven and earth, stands alone and does not change. Islam adds that this source is not a blind way but the Creator, who knows and wills. The Qur'an calls Him the One, who was not begotten, with none like Him.
The Tao produces all things, but it lays no claim to them and does not control them. So it is not a ruler who gives commands.
Rebuttal
A source that produces things, carries them through their stages and brings them to ripeness is acting, and acting toward a goal. Islam agrees that God (﷾) is beyond our grasp: the Qur'an says no eye can take Him in. But it adds that He knows all things, and has made Himself known by His most beautiful names.
Shinto: the kami came into being
Shinto honours the kami, the powers felt in nature, places and ancestors. Its oldest book begins with Heaven and Earth already there, and the first kami came into being when Heaven and Earth began. Proving one Creator is not part of the faith.
In the Kojiki the first kami came into being when Heaven and Earth began; no single Creator made them.
Rebuttal
Then the kami themselves had a beginning. And the Kojiki's preface speaks of chaos already forming before them. Whatever comes into being, and the chaos it comes from, both need a cause. The Qur'an asks of all such things: were they created out of nothing, or did they create themselves?
Anything with extraordinary power that inspires awe may be called kami, among them mountains, rivers and people.
Rebuttal
Awe is the right feeling before a mountain or the sea. But the mountain did not make itself. The Qur'an turns that awe toward the One who made the sun and the moon. It tells people to bow to Him, not to what He made.
The Islamic perspective
The universe began, so it has a cause outside time and matter, with immense power, that chose to create. That is God. The Qur'an asks people to reach this by reason, from the signs in creation.
Whatever begins to exist has a cause. The universe began to exist. So the universe has a cause.
A past with no beginning could never have been crossed to reach today. So the universe had a first moment.
The cause of the universe must be outside time, not made of matter, hugely powerful, and able to choose. A cause that always existed produced an effect with a starting date.
The Qur'an asks whether people were created from nothing, created themselves, or created the heavens and the earth. That leaves only a Creator.
The Qur'an says the heavens and the earth were once joined and then parted.
God (﷾) created the heavens and the earth, and His signs in them point to Him.