There seems to be a perception among religious people that atheists believe all religious people to be stupid, ignorant, or deluded. (Witness a recent question on the Yahoo Answers service.) Where this perception comes from is something of a mystery - given how severely underrepresented atheists are in the media, it is almost certain that it does not actually come from anything we have said. Nevertheless, there is no question that a large number of theists take the statement "Hello, I'm an atheist" as a personal insult. What can be the reason for this?
The most obvious answer, and the one I think many religious people themselves would give, is that atheism is an insult to the believer because it rejects the believer's most cherished principles of faith as untrue. This answer seems simple and logical, but I am convinced that it is not the correct one.
For one thing, if this were true, then believers should be equally offended upon meeting other believers who share none of the key tenets of their faith. But this does not seem to be the case. Although our heavily Christian society strongly distrusts atheists, it is not equally distrustful of other religious groups who reject fundamental Christian claims - for example, Buddhists or Hindus, who just like atheists reject Christian claims about the nature of the afterlife, the divinity of Jesus, salvation by faith alone, and the rules given by God for living. Nor do Christians seem to be equally offended by Jews and Muslims who reject the single most important part of all of Christianity, the incarnation and resurrection of the Son, as well as the trinitarian nature of God. Even groups within Christianity disagree fiercely about vital areas of doctrine, such as whether baptism is necessary for salvation, whether communion involves the literal or only symbolic presence of the divine, or whether the Vatican is a legitimate inheritor of Jesus' promise to Peter. But despite their vast differences, in general none of these groups seem to take the others' existence as a personal affront. This persistent attitude of distrust and dislike seems to be concentrated solely on atheists.
One might also suggest that it is the atheist's lack of belief in God that inspires resentment among theists. This idea does have the virtue of explaining why theists usually do not view other kinds of theists with similar suspicion, but I think it falters on similar grounds. Again, what can it possibly mean to say that two theists of different traditions "both believe in God" when they differ on every significant aspect of God's character? Are Brahma, Shiva and Vishnu really equivalent in any meaningful way to the Christian trinity? Do the Zoroastrians' opposing good and evil deities, or the Buddhist view of gods as finite beings subject to the law of karma just like humans, compare to the all-powerful god of monotheism? What about religious views that depict God as an impersonal, cosmic force, as opposed to ones that depict God as anthropomorphic and personal?
I have another proposal, which I believe explains the facts more convincingly. The religious dislike for atheists is not because atheists reject God - it is because we reject religion.
Despite their differing creeds, all religions are alike in that they give their adherents a sense of community, of belonging, and of identity: a social support structure, if you will. Atheists have nothing comparable, and this is easily interpreted as - in fact, in a way, it is - a statement that people do not need the structure of religion.
This is what angers and upsets believers who encounter atheists. They have devoted so much of their lives to their support structure, and their identity is so intimately bound up with it, that meeting a person who needs none of those things is naturally threatening to them, because it suggests that all their effort was unnecessary, even wasted. And from there, it is only a small step to the conclusion that atheists must think them deficient in intelligence not to have seen the better way, if there is one. In my experience, even people who care nothing for dogma and doctrine, who disagree with their church leadership on virtually every issue, often react with anger or incredulity to the suggestion that they leave their religion, because they have been brought up in it and participated in it for so long that it has become part of how they see themselves.
This hypothesis explains why believers are generally not offended by the existence of other believers: even if two groups of theists differ drastically about their actual beliefs, each can see that the other group provides its members with the same kind of social structure that their own religion provides them. In its way, this is a tacit affirmation that belonging to religious groups is normal and desirable and is how human beings are meant to live, and therefore reassuring. But the existence of atheists threatens to wreck this whole gentleman's agreement (and religion is very much a gentleman's agreement), and so it is no surprise that theists mistrust and fear us. They are offended and angered not so much by what we say or do, but by what we represent.
Is it possible to change this reaction? Certainly, if atheists want to be heard, we will have to improve our public image. But how can we do that if the negative perception of us stems from nothing we do but from our very existence?
Happily, the solution to this dilemma is the same as the solution to many other issues facing atheists: organize. Just because atheists reject the social structure of religion does not mean we lack any need to associate with others. No person, not even an atheist, is an island. Rather, we reject religion because a higher virtue, allegiance to the truth, compels us to stand apart. But so far, though many of us stand apart from religion, we have not taken the obvious step of standing together with others who do so. We can and should do this. Not only will it magnify the power of our individual voices, it may well improve our image in the eyes of the believing mainstream who see us as dangerous and strange iconoclasts.
I do not suggest that we form a "church of atheism" that mimics the tone and style of religious organizations without the content. That would strike me as silly and pointless, an opinion in which I trust I am not alone. But in addition to advocating political organization, I do not think it would be a bad thing for atheists to achieve a greater level of social organization as well (and of course the two are strongly linked). Why aren't there more atheist groups that can organize excursions to museums and science lectures, nature hikes, and other worthwhile activities? There is a whole world to know and experience, and atheists have every reason to want to drink as deeply from it as they can. And the more worthwhile and enjoyable we make our lives, the more likely it is that others will see this and want to join us!