The single most common argument for the existence of a single creator-god is that things cannot "create themselves", therefore, there must be a Creator1. Everything must have a cause, therefore, god exists as the 'first mover'. But the argument means that god itself must have a cause, else, god cannot exist. So the true form of this argument is that "everything must have a cause, except one thing". The problem with this is that it gives no indication of what the first-cause must be. Monotheistic religions such as Judaism, Christianity and Islam say that 'god did it'2. Karmic religions such as Buddhism and Taoism conclude that a set of atheist mechanistic rules govern creation and destruction. Secular atheists conclude that the Universe itself is the first-cause. In all cases, the first-cause is not a pure or simple concept, but a collection of well-developed characteristics, drives and powers. "Everything must have a cause" arguments do not bring us any closer to working out why anything exists, nor to what the cause is, nor if the universe is a theistic, deist or atheistic one.
People who believe in god (theists) subscribe to the theistic idea that the Universe was the work of a creator-god. They say that 'god did it'. Many argue that all things must have a cause, and therefore, god exists as the 'first cause' of everything else. These people ask questions like "Who do you think created you?" and "Do you think the Universe came out of nothing... who do you think created it?". These types of questions both rely on the "everything must have a cause, therefore God exists" argument.1,2
Some truly great thinkers in history have posited first cause arguments for a "designer":
“In like manner of reasoning, everything we behold carries in itself the internal evidence that it did not make itself. [...] In the first place, I know I did not make myself, and yet I have existence; and by searching into the nature of other things, I find that no other thing could make itself; and yet millions of other things exist; therefore it is, that I know, by positive conclusion resulting from this search, that there is a power superior to all those things, and that power is God.”
"The Age of Reason" by Thomas Paine (1807)3
The conclusion that "everything has a cause apart from one thing" is not only a cornerstone of atheism and theism, but also a generic scientific and religious theory. Many scientists (but not all) think that the big bang was a singularity. This means, it was not the result of an ordered physical law, but was the result of an impossible situation that no laws cover. In other words, it was uncaused. The religious theory that there is a "first cause" is found not only in monotheistic religions such as Judaism, Christianity and Islam, but in atheist religions such as Buddhism too - eventually, everything will be returned to Nirvana and the samsaric cycle of cause-and-effect will end, but, Nirvana itself is uncaused4. All around, the reasoning that "everything has a cause, except one thing" is used to support disparate and contradictory conclusions.
If everything needs a creator, a designer, or a cause, then, the only question of worth becomes "what created the creator?", or "who designed the designer?" or "what caused the First Cause?". If God had the willpower, desire, powers and intellectual capability to create the Universe, then, what created those traits of God, and what created God itself? It is very difficult to argue that everything has a cause without admitting that you need one thing at least that remains uncaused.
When you answer that secondary problem (who created the creator?), then, more questions arise. If you answer "the creator didn't need creating because it has existed eternally" then the question becomes "so who invented time?" or "who invented eternality?". If your argument is that "everything must have a cause", then very quickly you find yourself in the impossible situation of trying to explain how your theorized "first cause" does not, itself, actually need to be caused after all. This is what Bertrand Russell meant when he said that "all causal explanations must have an arbitrary beginning"5.
Therefore, first-cause arguments always must become "everything has a cause, except one thing". But this, more honest (but less clean) version of the argument provides no logical reason for concluding that any particular force or being is the 'first mover'.
“The argument that there must exist a first cause of everything is open to serious doubt as long as we adhere to any simple notion of cause, irrespective of whether the universe is infinitely old, or had a definite beginning in time.”
Because "everything must have a cause, except one thing" type of arguments cannot inherently tell us what the first cause is, we are free to use our imagination. Different religions and cultures lead people to make difference assumptions about what that first cause is. Here are some major contenders:
“The theist claims that God is the answer. But, then, why is there God rather than nothing?”
"God, the Failed Hypothesis: How Science Shows That God Does Not Exist" by Prof. Victor J. Stenger (2007)7
Monotheism: God, a complicated being with desires, powers, complex planning capability, knowledge and sentience, had no cause, and had no designer. This multifaceted being, with all its features, presence and prominence, magically came into being as it is, with no prior planning or control. Very lucky, really, that is decided to create life and be a good guy (if that's what you believe)! The monotheistic conclusion has no evidence to support it and it leaves the question of "but how could this being come to exist?" unanswered.
Eastern Karmic Religions: The Universe is a mistaken distraction from true formless, unconscious being, and the long machinations of karma will eventually resolve all constituent beings back into nirvana, a state of difficult-to-define nothingness. This atheistic line of thought has no evidence to support it and it leaves the question "but how could this happen?" unanswered.
Explicit Atheism: The Universe was created via the interaction of a few fundamental forces of nature, resulting initially in the Big Bang, and eventually in the creation of the hundreds of billions of galaxies, stars and worlds, a tiny fraction of which have come to support life. This scientific theory has concrete evidence for it, but, still does not answer the question of "but why do those forces of nature exist?" unanswered.
The biggest problem with the "everything has a cause" argument is that it doesn't admit that it must mean that at least one thing does not have a cause. And the biggest problem with the resultant argument that "everything has a cause except the first cause" is that it can give no indication as to what the first cause could actually be - theistic and atheistic conclusions are both equally possible answers.
The "first-cause" element is just one part of overall debates about if and how god(s) were needed in order for the Universe to come into existence. Here are some further topics: