In various forms, the god theory has been popular for some thousands of years. It served a useful purpose in past millenia, providing a shared belief that built cohesive communities, great art, and some soaring architecture. However, despite the theory being long obsolete, some adherents continue to believe it - confusing their need for comfort with an explanation for the way nature really works.

  These people generally accept certain scientifically derived theories (such as gravity) - those who don't generally don't live long - but refuse to accept many that question their worldview, such as biological evolution and plate tectonics. Their stance, based on blind faith rather than genuine rational inquiry, is problematic for any rational person attempting to explain the flaws in their reasoning, since they don't accept any real criticism of their worldview.

  This lack of rationality makes some believers in the god theory extremely dangerous. Wars have been fought and whole countries taken over by obsessive believers in the god theory, who then use violence and intimidation to impose their tunnel visions on others. (Hey, George W!) Rational thinking - being always open to question - tends to minimise these urges, and can keep the world somewhat safer. That's why all atheists have a duty to persuade believers in the god theory onto the better path of logic and rational thinking. Note that you shouldn't atttempt to deny anyone their private beliefs - that's just rude - but if they make the mistake of thinking their religious beliefs substitute or override the objective reality, you should gently put them right.

 To help, here's a handy list of reasons why the various god theories are wrong. Next time you're bearded by a fundie, remember that due to the very nature of religion - adherence to dogma rather than openness to question - these rationally-derived reasons will always be better than his god theory, no matter how violently he attempts to force his worldview on you. It may be cold comfort when a suicide bomber's about to frag you into small bloody chunks, but as you spray out into a billion scraps of bone, at least your last thought will be that all he did was prove you right.

  First, some basic concepts. The god theory exists for three reasons:

  1. To wield power over people.
  2. To satisfy a need for comfort.
  3. Pascal's Wager.

All are understandable. The power-crazed need methods to gain and retain power; the weak and tired need the comfort of believing in something simple. And Pascal's Wager - religious belief is a good bet just in case - is only flawed due to the broad range of deities available: what if god turned out to be a big chicken and you've worshipped a guy with a beard for decades? Now, on to the reasons:

  Reason No. 1: This is the biggest ,and the only one that really matters: science and religion are not equals. Religions are just worldviews, the private hunches of a group of people. They are, therefore, hypotheses, and bad ones at that. The purpose of science is to explain and disprove hypotheses to turn them into tested theories, not to present some sort of competing viewpoint.
  Science is not a worldview; it is a method for verifying and falsifying worldviews. Since no god theory is supported by testable evidence, god theories can be discounted. All religion is just a small and discredited subset of the field of scientific enquiry.

  Reason No.2: There is no evidence for the god theory. Thousands of years ago, we peopled the skies with supernatural beings to explain how the world worked; today, we've filled in many of the gaps in our knowledge and god theories are no longer needed. (Even the origin of the universe isn't the complete mystery it once was.) But thanks to power structures like the western churches, plenty of people have a vested interest in your continued belief. That's the only reason the god theory persists: it's a form of control.

  Reason No.3: God theorists use selective evidence. God theorists like to bend logic to give the appearance of rational enquiry - treating science as a poll to be argued rather than a truth to be discovered. If a piece of evidence - such as those dinosaur and human footprints apparently made around the same time in New Mexico - fits their pet theory, they'll take it on board. Don't be fooled - there will always be a context it's removed from. (Those footprints were later proved to be separated by tens of millions of years - but plenty of creationists still put them forward as support.)
  Conversely, god theories tend to put forward showy individual examples as proving a whole class. The human eye is often used - how could anything so useful have evolved from scratch? - but the eye's evolution is completely consistent with Darwin (and is in fact one of the simpler organs.) God theorists are showmen, not scientists.
  God theorists also deny a basic part of rational enquiry: when searching for universal laws, even one contrary example disproves the law. A single apple hanging unsupported in the air would disprove 400 years of Newtonian mechanics, and gravity would have to be rethought from scratch. (Actually, this happened in the 1910s, except the apple was called Pluto and Newton's Laws still held to an accuracy of 1 in 10^7.)
  This picking and choosing by god theorists smacks of desperation - it's the logic of a lawyer, not of rational enquiry into nature. The god theory fails another basic test.

  Reason No. 4: The universe is hard to understand. Science - particularly the bits that involve quanta, extra dimensions, or strings - is difficult. Too much so for most people, and too time-consuming to get to grips with; objective truth is just beyond most people's time or capacity. Much easier to believe some guy sitting on a cloud did it all.
  But 'easier' does not equate to 'correct'. Many people draw great warmth and happiness from their religion; that's fine, it's what religion is for. And if they're happy living in that bubble, let them. But don't let them push the fiction that their little warm bubbles represent rational thinking. It's just their defence against having to do any hard thinking.
  
  Reason No. 5: We don't need any god theories. We've discovered no law of nature that needs something supernatural layered on top to make it work; nature is wonderful enough already. Discounting the various god theories is an unavoidable consequence of even a cursory knowledge of nature.
  Religion is superstition, not a rational worldview. Believers in the god theory often pit scripture against science, as if they were somehow equivalent (or as if science needed some 'other point of view' to balance things up.) This is wrong. Religion is dogma, something to be believed in rather than questioned; science is questions, a method and approach rather than a body of knowledge.

  Reason No.6: God theorists confuse subjectivity with objectivity. The god theory is not an explanation for how the world works; it is at core a need for comfort, an emotional anchor in a complex world. The god concept is a simple imaginative construct no different to a soap opera plot; it gives comfort and a sense of belonging. But however nice it feels, feelings can't represent how things work. Religion is subjective; rational scientific thinking is objective. (Individual scientists may be subjective - but ultimately, they've got to believe the results of an experiment and hard evidence, not their pet hypothesis. Nature cannot be fooled.)
  That's why atheists get so angry when a god theorist asks them to 'keep an open mind' about the god theory. Is his mind open? An atheist isn't making any unusual claim; he's just accepting the world on the basis of rationally thought out theories supported by evidence. All god theories are very unusual claims, and as such require unusual levels of proof - and no proof has ever been presented. Once again, the god theory can be safely discounted.

  Reason No.7: If a theory isn't open to question, it's not a theory at all. Believers in the god theory never accept the possibility that their theory could be wrong. All rational thinkers, however, are constantly open to this possibility. Much of science is dedicated to finding holes in other people's theories; the moment one scientist makes a hypothesis, hundreds of others try to prove it wrong. In other words, all established scientific theories have been tested to their limits and have a mountain of evidence to support them - but all are weak. Even one piece of testable evidence against a theory is enough to prove it wrong.
  Any god theory's basic tenet is that it relies on faith and is not open to question - and is therefore weak by design.

  Reason No.8: The universe doesn't need 'purpose' or 'meaning'. Believers in god theories often ask nonbelievers what the universe is for, if not to demonstrate some conscious mind behind it. They then put forward the god theory as answering this straw man question.
  But why should the universe have, or need, any purpose or meaning? It's just a collection of interactions arising from the properties of nature; to think otherwise is to suggest there's something special about the human species - a few individuals on a lump of rock circling a single one of countless suns in a billion galaxies. Purpose and meaning are human inventions; the universe has no meaning besides that which we choose to impose on it.

  Reason No.9: A theory isn't true just because lots of people believe it. Objective truth is not opinion polling. How many people really understand quantum mechanics, or Einstein's Relativity, or even Newton's equations? Yet each has a mountain of evidence supporting it. While our understanding of science remains incomplete, nothing we've seen so far suggests a god theory has any role in explaining anything. If a god theorist protests, "But over three billion people believe in a god!", your riposte should be that gravity, electromagnetism, and the strong and weak nuclear forces apply to all of us, regardless of whether we believe them or not.

  Reason No.10: Mankind is not special. All life springs from a very few humble beginnings; it's written there in the map of our DNA, linking us to every other life form on earth. We're reading the tree and there's no longer any room for reasonable doubt about this interconnectedness: there's nothing separating us from other beasts, nothing to mark us out as special. No need for a god there. All religion is just a scam to make people think there is.

  Reason No.11: Religious texts are full of easily-checked errors. The Bible and the Koran are riddled with simple factual errors, such as the number of legs on a spider. Yet they're supposedly written by an all-powerful supernatural creature?! So the god-theorists' infallible protagonist has already been proven fallible. Therefore if they can't get the little details right, the god theory is also likely to be wrong on the big stuff. Like many other texts down the ages, these books were written as a technique of control, and 'revealing the word of god' was the ideology used to imbue them with power.
  Scripture is full of illogical constructs. Think about this: if a life on earth is finite, there's only a finite amount of bad things you can do here. Yet in some religions you'll be punished by - eternal damnation? An infinite amount of punishment? Religious texts lack intellectual rigour, and therefore are not serious competitors to science.

  Reason No.12: 'Intelligent design' is anything but. If some intelligent entity designed us, he did a really bad job of it. Look at the human body. Eating through your breathing hole? And let's not get into the lower regions - excreting and reproducing via the same organs? Yeuch. Even this basic examination is complete evidence against intelligent design; it demonstrates that living things are the result of natural selection on diverse populations, and nothing else. Evolution looks zero steps ahead. That's why evolution is messy and diverse; it's a random process of mutation and mixing winnowed by nonrandom natural selection.
  Even intelligence does not make us different. We're smart, sure, but intelligence is just another evolved trait, like opposable thumbs. It evolved to deal with unpredictable environments.
  And anyway, life's not that much of a mystery - and it gets clearer every day. We can mix chemicals and get amino acids, build them into protein strings, part-predict their folding and part-understand how they switch direct the growth of cells. We've got a reasonable understanding of how life works. So here's the point: if life can be mixed up in a lab, the supernatural creatures of the various god theories aren't any smarter or more powerful than us - hardly worthy of a god theory at all, really.

  So there you have it: a few handy rebuttals for dealing with religionists. Use and enjoy!