I have been debating with religious apologists for as long as I have been an atheist, some 23 to 24 years. Over that time, I have come to recognize certain common fallacies in their arguments. What follows is a list of the most common logical fallacies committed by religious apologists:
1. Psychogenetic Fallacy. This logical fallacy states that if someone has psychological motivations for putting forth an argument, then he's biased, so his argument must be wrong. For instance: "He was molested by a priest. That's why he thinks there's no God." Psychological motivations give you information about the proponent of the argument, but they tell you nothing about the truth of his proposition. I can hate Christianity because it condoned slavery and because most racist organizations in America are Christian organizations. That does not subtract at all from my legitimate, logical arguments against the plausibility of an all-knowing, all-powerful, omni-benevolent creator.
2. Ad Ignorantiam is an argument from ignorance. The fallacy that a proposition is true simply because it has not been proven false. I'm sure you've all heard theists try to counter the argument that they have no proof in God's existence with "Well, you can't prove he doesn't exist!" This is silly of course because it is nearly impossible to prove a negative. I can't prove that fairies, elves, Leprechauns, vampires, and werewolves don't exist either, but I think we'd all agree that you'd have to be an idiot to believe in any of them because of the improbability of such beliefs.
3. The Teological Argument or Argument from Final Consequences is based on a reversal of cause and effect, because they argue that something is caused by the effect it has, or purpose that is serves. For example: "God must exist, because otherwise life would have no meaning." or "God must exist or else there could be no absolute moral laws." This only demonstrates that the conclusions are distasteful to you, not that they are false. Life may not have an objective absolute meaning or morality outside of that which we ascribe to it. This is not only possible, but plausible. Life's only meaning may be to exist and then cease to exist so that something else may exist. This may be a depressing, unpalatable answer, but it might also be true.
4. Least Plausible Hypothesis. This is a fallacy in which the most reasonable explanations are ignored in favor of a less plausible one. "I was praying for God to help me get this job and that night, the phone rang with a job offer." Obviously, this ignores the more plausible possibility that the prayee was the most qualified applicant.
There is an old rule for deciding which explanation is the most plausible by favoring the one that relies on the least number of unfounded assumptions; the simplest explanation. In essence, it is the principle that the simplest explanation is usually the correct one. It is most often called "Occam's Razor". To put it another way, just because you smell smoke doesn't mean there are dragons about. This is one of the reasons that I reject the absurd "Liar, Lunatic or Lord" argument. Obviously, liar or lunatic conform much better to the principle of Occam's Razor than "son of an omnipotent, omniscient, creator god, sent to earth to suffer and die for man's sins."
5. Argument from Incredulity or Argument from Astonishment. This is another form of The Argument From Ignorance. It is an intellectually pessimistic argument that says that if I don't know something, then no one else knows, and no one ever will. It confuses "currently unexplained" with "unexplainable". Because we do not currently have an adequate explanation for a phenomenon does not mean that it is forever unexplainable, or that it therefore defies the laws of nature or requires a paranormal explanation. An example of this is the "God of the Gaps" argument of creationists which seeks to label anything we cannot currently explain as "unexplainable" and therefore an act of god. William Lane Craig commits this particular fallacy when he argues for Intelligent Design by stating that the hyper-dense singularity that exploded to create all matter in the universe (The Big Bang) must have exploded for a reason and, since science has yet to come up with a definitive explanation for what caused The Big Bang, it must have been God. Obviously, by that reasoning, we could say: "Since we don't know what caused The Big Bang, it must have been giant cosmic chipmunks or gremlins or leprechauns." Again, this is just idiotic reasoning.
The other issue with the "God of The Gaps" is that as the gaps in our understanding slowly close, God must necessarily become smaller and ultimately disappear. One example of this is the attempt to refute the Theory of Evolution by pointing out the gaps in the fossil record. As those gaps are filled, (and they are being filled consistently year after year) an already weak argument weakens further.
6. False Dichotomy. Arbitrarily reducing a set of many possibilities to only two in order to make the conclusion appear obvious when it is actually far more complex. For example: "Jesus was not a liar, therefore he must have been God." assumes these are the only two possibilities when obviously there are many other possibilities like Jesus being confused, mistaken, or even deliberately manipulated by those around him. The famous "Liar, Lunatic, or Lord" argument is a false trichotomy for the same reasons.
7. Non-Sequitur. In Latin this term translates to "doesn't follow". This refers to an argument in which the conclusion does not necessarily follow from the premises. In other words, a logical connection is implied where none exists i.e. "There is so much beauty in nature. There must be a God."
8. Reductio ad absurdum. In formal logic, the reductio ad absurdum can actually lead to factual conclusions. It follows the form that if the premises are assumed to be true it necessarily leads to a ridiculous conclusion therefore one or more premises must be false. The fallacious form of this argument refers to the abuse of it by stretching the logic in order to force an absurd conclusion. Christian Apologists often commit this fallacy by arguing that if I don't believe in anything I haven't seen then I must not believe in Antarctica or The North Pole because I haven't seen either place with my own eyes. This is a false reductio ad absurdum because it ignores other ways that humans acquire knowledge besides personal experience, i.e. the documented and confirmed experiences of others, photographic evidence, and other forms of empirical evidence. I haven't seen Antarctica, but I have seen polar bears and penguins. In short, doubting the existence of God does not lead to the conclusion that one must also reject the existence of The North Pole or Antarctica.
9. Slippery slope fallacy. This logical fallacy is the argument that a position is not consistent or tenable because accepting the position means that the extreme of the position (however ridiculous) must also be accepted, which would inevitably lead to harmful consequences. "If we let two men marry what's to stop a guy from marrying a horse if he wanted to?" Any reasonable adult, outside of the Christian Right, can see the obvious absurdity in this reasoning.
10. Straw Man. Arguing against a position, which you create specifically to be easy to argue against, rather than the position actually held by those who oppose your point of view. My aunt once forwarded an email about an atheist arguing with a Christian that he didn't believe in Jesus because why would Jesus lower himself by consorting with humans and becoming human. It employed some clever metaphor. I believe it was a man trying to get chickens to follow him into a chicken coop by acting like a chicken. All the Christian who read it applauded the witty way in which the protagonist of the tale countered the atheists argument, but it was a ridiculous argument to begin with and no atheist I know would make an argument like that. There are dozens of strong reasons for not believing in the Christian deity and I have never heard an atheist say he didn't believe because God lowered himself to man's level when he became human. It was one of the sillier Straw Man arguments I'd heard.
11. Tautology. A tautology is an argument that utilizes circular reasoning. The conclusion is evidence of its self. As one blogger put it:
"God is Good because the bible says that God is good and the bible was written by God who would not lie because he is good."
12. Argumentum ad populum. This is a fallacy that assumes that if a majority of people hold a certain belief, that belief must be true. It attempts to turn truth into a popularity contest where the conclusion with the most people believing it to be true is therefore true. Obviously, truth is not decided by majority vote. "People all over the world, in many different cultures believe in the existence of God. They can't all be wrong!" Well, of course they can all be wrong. There was a time not long ago when the majority of people believed that demons caused illnesses and the world was flat.
13. Unfalsifiable Hypothesis/ Special Pleading. An unfalsifiable hypothesis is exactly what it sounds like, a theory that cannot be disproven. Ask a theist how they would know if God did not love them, what evidence they would need to prove that God in fact hated them and they could give you no answer because their belief that God loves them is not based on evidence therefore no amount of evidence can refute it. By this reasoning, if they were to become poor, crippled by illness, victimized by criminals, and see their loved ones tortured and murdered before their own eyes, they would still believe that God loved them because their belief in God's love is not based on empirical facts.
14. Argument By Selective Observation. The Argument By Selective Observation "cherry picking" as it is more popularly known, is the enumeration of favorable circumstances while ignoring the unfavorable. "Jesus was the God of Peace! He told Everyone to love thy neighbor as thy self." He also condoned slavery and condemned anyone who did not believe in him to death. But let's ignore that.
15. Ad hominem. An ad hominem attack is an attempt to refute an argument by attacking the argument's proponents, rather than addressing the argument itself. Theists often commit this fallacy, avoiding the arguments of freethinker by accusing them of being "Angry at God" or of "Just wanting to live immoral lives and sin without consequences". Atheists, on the other hand, have also been known to commit this particular fallacy, for example, by calling people who believe in Christ crazy or stupid. I know, I've been known to call believers idiots as well, but I counter their arguments first and don't use "Theists are idiots" as the counter argument.
So there they are, my top fifteen. The next time you argue with a theist, see how many of these fallacies you catch them using. If they're an apologist worth their salt they'll hit every one.