1. "You can't prove god doesn't exist."
This is true. However, I cannot prove that leprechauns do not exist either. We both acknowledge that the absence of any evidence is completely predictive of non-existence in the case of leprechauns. Indeed, if something didn't exist, what more evidence could you have than the lack of any evidence for its existence?
The fact that I cannot prove god exists by no means indicates that you have a good reason to believe he exists. This argument is bested completely by Bertrand Russell's famous teapot argument:
If I were to suggest that between the Earth and Mars there is a china teapot revolving about the sun in an elliptical orbit, nobody would be able to disprove my assertion provided I were careful to add that the teapot is too small to be revealed even by our most powerful telescopes. But if I were to go on to say that, since my assertion cannot be disproved, it is an intolerable presumption on the part of human reason to doubt it, I should rightly be thought to be talking nonsense. If, however, the existence of such a teapot were affirmed in ancient books, taught as the sacred truth every Sunday, and instilled into the minds of children at school, hesitation to believe in its existence would become a mark of eccentricity and entitle the doubter to the attentions of the psychiatrist in an enlightened age or of the Inquisitor in an earlier time.For those making claims in any other discipline, they are immediately compelled to provide their reasons for believing as they do - and their reasons (or lack thereof) would be the ultimate indicator of how likely their beliefs are to be true. If somebody believes the moon landing was a hoax, they will immediately begin spouting off their (bad) reasons. Even those people would never be tempted for a moment to say they believe the moon landing was a hoax, despite all the evidence that it happened, simply because they have faith that the moon landing never happened. Furthermore, if they did decide to use such terrible arguments, they would be immediately and rightly dismissed.
We should do the same with people evincing these arguments in defense of what they "know" about god.
2. Kalam Cosmological argument (everything that exists needs a creator).
People using this argument already believe in a god that exists without a creator. So, as long as they believe that something can exist without a creator, it doesn't necessarily follow that whatever exists without a creator has a mind or a conscience. In fact, we already have matter and the laws of physics, which means that adding the additional variable of god violates Occam's Razor and favors a universe that has always existed.
3. Teleological argument or "argument from design" (look around you, you think all this just happened?).
In 1802, William Paley authored an argument in his book Natural Theology that would come to be used by Christians and other religious people up to this day; the watchmaker analogy:
In crossing a heath, suppose I pitched my foot against a stone, and were asked how the stone came to be there; I might possibly answer, that, for anything I knew to the contrary, it had lain there forever: nor would it perhaps be very easy to show the absurdity of this answer. But suppose I had found a watch upon the ground, and it should be inquired how the watch happened to be in that place; I should hardly think of the answer I had before given, that for anything I knew, the watch might have always been there.Better than I ever could, Richard Dawkins has eviscerated this argument in his book The Blind Watchmaker.
The idea of the watchmaker argument is that order is resultant of a designer. This argument falls flat on its face when you realize that nature produces extremely complex order on its own. The solar system in which we now live is the product of matter - that's it. Matter bends space time and produces the effect we call gravity, which forms big enough chunks of matter into round objects called planets, moons, or stars. It also causes those objects to orbit larger objects. If you have enough matter, a highly ordered system like ours is the inevitable result, with no appeal to god being required. The physical properties of matter, and nothing more, take care of all of it.
So what religious people invoking the argument from design will do, assuming they care enough to modify a defeated argument rather than falling back on faith, is to say that order that stands out from the order around it requires a designer. Of course, they have no reason to believe this, but some will try it anyway. What they do not realize is that the argument does not make any allowance for order created by nature - it claims that natural order was created by god, which we know it is not.
4. Religion serves a useful purpose (it's comforting, makes people better, etc). In this, I'll also tackle "My faith doesn't hurt anybody" and "Atheism makes people do bad things too!"
It should first be noted that this is not an argument supporting the truth of a religious claim. Even if Christianity were the only stabilizing influence we had, that would only mean that we required the belief - it would have no bearing on whether or not the belief were true. Santa Clause is a stabilizing influence on the behavior of children, but that does not make it even a little true.
However, there is no useful purpose scripture serves that cannot be found within reason. Atheists are moral people. What's more, a non-believer is compassionate out of sheer empathy, rather than a fear of punishment or a lust for reward. On the other hand, consider the implications of the claim that we need belief in god to be moral. Who could possibly say this other than people who would be raping, lying, or stealing pornography without the belief that they will be punished for doing so? In this, it seems atheists are at a higher stage of moral development than people making the argument that morality requires god.
Another version of this claim is that belief in god gives people hope. Of course, this does not make it true - in fact, consider how other people invoking this argument would sound. What if somebody told you that they believed that faeries will one day emerge from their garden and give them eternal life, if only they enrich the soil by burying a $20 bill every week? This belief would offer a tremendous amount of hope, but who else but an idiot or an insane person would actually believe it on those grounds? In our hypothetical gardner's case, they're only losing $20 a week, but similarly crazy beliefs in scripture come with more significant behavior resulting of the necessary divorce from common sense these religions require. As Sam Harris puts it:
There are, at this very moment, perfectly ordinary Shia and Sunni Muslims drilling holes into each other's brains with power tools in the suburbs of Baghdad. What are the chances they would be doing this without the "benefit" of their incompatible religious identities?It is at this time that a believer may be tempted to say that atheism also makes people bad, such as Hitler, Stalin, and Pol Pot (it should be noted that Hitler was a Roman Catholic, but religious people will still include his name in this list). While we should be eager to admit that some bad people do reject the idea of god, one can only wonder how this actually was responsible for their evil. For instance, Hitler and Stalin both had mustaches - how do we know it was not their mustaches that turned them into monsters? All three of these men rejected the existence of Zeus, could this have been the fount from which their malice poured? What about unicorns? I'm sure they all deny the existence of such mythical beasts. In the cases of Stalin and Pol Pot, was their disbelief that Jesus truly rose from the dead really the mother of their wickedness? It seems clear that their deeds were resultant of bad reasoning rather than skepticism.
To be an atheist, one must only believe that there is no god. That's it. There are no doctrinal beliefs or prescribed behaviors associated with the word. The behavior of atheists is bound by what is reasonable and, since many people are unreasonable, there are some unreasonable atheists in the world. That is the primary problem with faith: how much it encourages unreason. In fact faith, being the defense for a belief when one has no tangible explanation for why they believe it, is fully dependent on abandoning good reasoning. Any sensible man condemns irrationality whether it comes from non-believers like Stalin and Pol Pot or believers like Hitler and George W. Bush - irrationality is bad no matter who embraces it.
So the best you could do with trying to tie people like Stalin to atheism is to say that their actions were particularly reasonable and therefore represented the end game of skeptical thinking or incredulity. Clearly, their actions were unreasonable and cruel, and virtually every atheist will join you in in adopting that fact.
5. "Science/reason is just another faith!"
First, this attempts to assert that all beliefs are equally likely to be true, which is clearly ridiculous. While the claims made by science are certainly beliefs, they are beliefs that are supported by observation, reason, and evidence and, therefore, more likely to be true than other beliefs. As an example, consider the "faith" that a dropped stone will fall to the Earth versus the faith that a dropped stone will soar into the stratosphere. One of these beliefs is in-keeping with everything we know, while the other is not. Because of this, any intelligent person must accept the former as being true until other evidence arises to make us think otherwise.
Science works under only one assumption: that the universe obeys a set of rules. If this assumption is true, then we should be able to observe the behavior resultant of these rules, such as the stone falling to the Earth.
But we can do so much more with science! When we observe our stone falling, we notice that it falls more quickly as it approaches the Earth. After making this observation, we can generate equations to explain this phenomenon and use them in the future to predict how objects will fall. Guess what: it works! As a matter of fact, it works so well that our current understanding of gravitation allows us to fire objects with uncanny accuracy to certain points even beyond our solar system. This confirms our assumption and very, very strongly suggests that reality is following a set of rules. Moreover, it indicates that we can deduce them.
Take a moment and just think of a few of the things we have acquired by affirming that the universe follows a set of discernable rules: cars, planes, the mouse on your computer. The monitor you're currently looking at represents our understanding of electromagnetism. Science has given you your cell phone, your mp3 player, plumbing, light bulbs - the list goes on and on.
Now, if your computer breaks, how will you fix it? Will you pray it back into operation? You could try, but I'll bet dollars to donuts that eventually even the Pope would break down and call somebody who understands the rules of the universe operant in making a computer function. All around us is a harmony spun by the fruits of science - we are literally drowning in the proof of its truth. We do not need faith.
These are the kind of demands we make of science - it must make predictions that work, it must explain things. We have been trying to scientifically confirm miracles for hundreds of years now, and have never managed to do so once (though we've exposed an ever-growing number of frauds). We would love to confirm an instance where prayer healed an amputee or where we could observe something distinctly god-like, but it has simply never happened.
So science is not in any way a matter of faith. In all actuality, faith is nothing more than failed science. The claim that a man can walk on water is a claim about physics; the claim that a man rose from the dead is a claim about biology - they are just claims that have never met the scientific requisites of evidence. We would be open to them, but if god exists he has given us more evidence that the universe is obscenely old by human standards than prayer is effective. In fact, Christians revel in believing claims that contradict the rules of the universe, calling these unsupported claims "miracles." It worries me to note that if believing events occurred in which the rules that govern reality were broken, with no good reason for doing so, is thought to be a magnificent glory, then what could possibly convince somebody that Christianity is complete and utter bullshit?
6. "You are just caricaturing faith!"
According to the World Christian Encyclopedia, there are over 34,000 different sects of Christianity in existence. Even if I were trying to caricature Christianity, how could I possibly do it with so many different voices claiming to have a deeper claim to truth than all the others?
I think it's fair to say that almost all Christians believe that Jesus was the son of God and that, at the very least, he rose from the dead. These claims are not more or less accurate depending on the nuances of the people believing it. All Christian sects are equally lacking of any reason to believe their core claims. Moreover, no Christian sect uses a bible that makes sense - which could be a very good reason why there is so much disagreement within the Christian community about what god's will truly is.
This statement also ignores the vast number of people who DO read the bible and take it word for word. Read the exit polls of relevant votes (here are just a few: Link 1, Link2, Link 3, Link 4, Link 5) if you doubt their numbers. So while moderates will cry that extremists are only a handful - a minority that gets all the press; they are apparently a minority that also casts all the votes.
Ultimately though, we should be focused on determining whether or not Christianity is true, and a religious moderate has just as much reason to believe that Jesus is the son of god as the most strident fundamentalist: zero. The only reason it would matter that I was caricaturing faith is when I point out the blatant evil that practitioners of the doctrine enact out of faith, which I cover in point #4. Moreover, if the claims that all Christians subscribe to are false (which all evidence would suggest they are), then what does it matter if I did happen to be wrong about how much damage faith does?
7. "How is my faith your business?"
Because we all have a responsibility to have good reasons for what we believe. Beliefs are the gatekeepers of our actions, and your actions affect those around you. If we do not have good reasons for believing the things we do, our good intentions can become terrifying.
Instances of religious people failing in the epistemological responsibility are everywhere, from parents denying their children medical assistance out of deference to faith to people flying planes into buildings because they believe it is the best thing possible.
We all acknowledge that it is alright to criticize this failure when we criticize peoples views on politics and policy. In every other situation, it is acceptable (even sensible) to criticize people for failing to hold good reasons for their beliefs, as we realize that people vote and act on those beliefs. For some reason, religion has earned a free pass on this that it does not deserve. In fact, because religions tend to encourage us to ignore evidence (they call this "faith"), it seems obvious that religion should be criticized more than other subjects.
Many religious people will say that their personal faith doesn't harm anybody, and most of them are right (although, they still voted based on what god supposedly does or doesn't like). However, while not all religious people are equally dangerous, all religious people equally fail in their responsibility to have good reasons for what they believe, which results in the instances of horror driven by unreason that do so often emerge from the faith community.
There is no reason to believe in things on bad or no evidence. It gets us nothing good that can't be found (easily) in rationality, while it produces the unreason that results in good people often doing bad things.
8. "What about moderates and Christians who are rational in all but their belief in god?"
9. "God exists because I feel him."
10. "We can't be moral without god."
11. "This nation was founded on Christianity."
12. "Without God, life has no meaning/purpose."
13. "God does not exist in our plane of existence."