Sorry for taking so long to get this. Today was the first day I've been at home in six weeks. Without further ado:
"Let me try to shine some light into this for at least one atheist - me. I want you to close your eyes and pretend that you are just an average guy. Now, I want you to imagine that you live in a society in which the overwhelming majority of your peers believe that eating grapes is evil, and that a gigantic monster will tear you limb from limb if you even think about eating grapes. But all is not lost! If you manage to go through life without giving in to their tastiness and juiciness, you'll get to see a beautiful green unicorn right before you die.
How does everybody know that this is true, Missionary? Well, that's easy. Some desert nomads from the bronze age wrote it down - well, that's not accurate. The desert nomads met the monster and saw the green unicorn, and then after generations of playing the 'telephone' game, somebody wrote it down. Then after being passed through the hands of thousands of people with their own agendas, biases, and transcription errors, it got translated into your native language. Isn't it easy to see why it's true?
And people really do believe that it is true. There have been wars fought between anti-grape groups that think that the monster eats you if you eat any grapes and anti-grape groups that think the monster only eats you if you eat the purple variety of grape. There are sub-sects which think that seedless grapes are especially abominable. There have been murders, tortures, and brutal religious-violence over whether it is even appropriate to touch grapes - including long-standing squabbles in which children too young to even know what a grape looks like have been used as living grenades to carry explosives to the 'heretics' on the other side of some anti-grape related issue.
It's not a problem for you when you don't see any problem with eating grapes, and you didn't mind so much when people laughed at your disbelief in the monster that'll get you for eating grapes. The fact that you eat grapes all the time and there haven't been any monster sightings doesn't seem to dissuade people. In fact, the troubling absence of monster sightings in any time since the dawn of recording devices doesn't seem to dissuade people either. Curious. You note the bizarre fact that the 'reward' for avoiding grapes isn't really all that awesome (so you get to see a green unicorn? That's not so great.). You also find curious the fact that the 'reward' comes at a time when it it completely unverifiable, so even if somebody did see the green unicorn, they'd never be able to tell you about it. None of these observations makes any lick of difference to anybody else, but who cares, right? If they want to believe in a weird story, that's their issue, not yours, right?
Well, not quite. See, you sent your kid to school with some grapes in his lunch and he got beat up by other kids who were afraid of the monster. You complained about it to the school, but the school administrators who were scared of the monster just as much as the kids refused to punish the kids who beat up your child. Every week, people concerned about your grape-eating habits drop by your home to leave you ridiculous pamphlets with crude drawings of what they think the monster might look like, and to point out how awesome seeing the green unicorn would be. You're polite to them and tell them that you're not really interested.
When your family found out that you were a grape-eater, they kicked you out of the house, even though you were young and your older sister got to live at home for another two years. You mother and father refused to have anything to do with you, and your father even contacted his attorney to have his will redrawn so as not to include a son who doesn't believe in the green unicorn. Your mother refused to attend your wedding - either because you were going to omit mention of the evilness of eating grapes or because you would mention them and sacreligiously just pretend that you thought they were evil to try to keep the peace. Despite your best efforts, when your children were born, every family member, neighbor, and 'well-wisher' tried to overrule your moral education of your child by telling them stories about what harms the monster would do to them if they followed in your footsteps, and how sad it would be that 'Daddy' doesn't get to see the unicorn.
Still with me? Now, you spend a lot of your time doing good things. You give blood regularly - saving dozens (maybe hundreds) of lives over your adult life. You regularly volunteer for charitable causes. You give up your Saturday mornings to pick up trash in a park, mow your elderly neighbor's lawn, or stuff lunches in bags for area homeless people. You give away for free professional advice that your work colleagues charge $200/hr for, simply because the people that you help need to be helped and they can't pay. You've never stolen anything (not even a pack of gum from a supermarket), and you do your best to avoid lying and cheating. Heck, you even avoid drinking and smoking just because you're not sure whether they're wrong or not.
Despite all this, the non-grape-eaters automatically assume, a priori, that you are on the verge of murdering them, molesting their children, or that every word that comes out of your mouth is a lie.
To make matters worse, there were a couple of mass-murderers about a century ago who didn't eat grapes. They killed a lot of people. Despite the fact that these mass murderers only killed prostitutes, and wrote about their hatred for prostitutes, non-grape-eaters smugly point to the mass-murderers and insinuate or even openly defend the idea that the mass-murderers killed prostitutes because they ate grapes.
You tried to coach a little-league team, but when word got around that you eat grapes, parents refused to let their children play on your team, and for the childrens' sake, you resigned and let another, less-competent, person coach the team.
You've dated women with whom you were madly in love, only to have every last one dump you over the fact that you eat grapes and don't believe in the green unicorn or monster - and do so completely self-righteously, since once they find out you're not a non-grape-eater, you're not really worth of any sort of consideration as a human being.
You are well trained in your chosen profession - you graduated with your degrees with honors and fielded offers from prestigious companies. Nonetheless, you are forced to hide your grape-eating (even performing sweeps of your online data to clear them of any grape-eating references), else your clients might go elsewhere. And it's not like you've not heard the whispers at previous jobs about your grape-eating, and had questions about the curious timing of your being necessarily downsized during a growth year right after rumors of your grape-eating swirled around the office.
Despite the fact that your society's foundational laws make it clear that the issue of grape evilness/non-evilness isn't something the government should get involved in, there's an annual governmental celebration of the truth of grapes' evilness and the reality of the green unicorn. Your currency is stamped with the slogan "Don't eat grapes!" And as if that weren't weird enough, people are ready to accuse you of being unpatriotic if you don't publicly pledge your fear of the monster.
Now, let's add to this idea. Suppose that for almost every 'electable' position you could imagine, whether that position is a political one, or just 'class treasurer,' you'll almost never get voted in. Your qualifications are almost irrelevant - you eat grapes and that's enough. It doesn't matter how bumbling your opposition is - all they have to do is swear up and down that they've never had grapes and never will, and they'll beat you, hands down.
Are you getting the picture here? Now, let's suppose that heretical scientists from other countries where the belief in the monster and green unicorn isn't so prevalent are making fascinating discoveries. It turns out that eating grapes can actually cure some diseases long-thought to be incurable. They're not quite to the point of giving those discoveries a lot of certainty, but early data looks amazing. With a little research, millions of people's lives could be improved. Unfortunately your politicians, whose overriding criteria for office is belief that eating grapes is evil, ban funding of research aimed at grape-eating benefits.
Now, suppose that after enduring constant stares, belittling comments disguised as 'concern,' and outright hostility for something as innocuous as eating a grape now and then, you find a place online where you can just be yourself and talk about things with other grape-eaters. You don't even have to talk about grape-eating. Politics, science, music, or even just posting pictures of yourselves is a fun way to pass the time without having to deal with the hassle of people going on all the time about how awful grape-eaters are.
Now, suppose that after having found this place, somebody comes along, accuses grape-eaters of being immoral, non-caring people. Suppose that he repeats the tired phrases about the gigantic monster whose going to eat every last one of us grape-eaters, and how awful it is that we simply aren't open to the possibility of the green unicorn. Suppose that he insinuates that people who don't believe in the green unicorn do it because they are angry at the unicorn, or that they eat grapes just because they want to be evil. Suppose that this individual, as can invariably be expected, brings up the mass murderers from a century prior that killed prostitutes and holds them up as examples of grape-eaters. Suppose that after being rebuffed about these things, the anti-grape-eating advocate chastises the grape-eaters for being closed-minded and unwilling to listen to things that most every last grape-eater has heard, thought about in substantially more detail than the anti-grape-eating advocate, and dismissed. Suppose that for evidence of the truth of his claims, the anti-grape-eating advocate cites the words of the desert nomads, written down generations after they were actually uttered, and then complains when the grape-eaters don't treat the words of bronze-aged desert nomads as being all that powerful as evidence.
Suppose that to rebut these claims, the anti-grape-eating advocate points out that these same documents tell about how early believers in the monster were killed by others, and that they wouldn't have died for this belief if it were false - then suppose that the anti-grape-eating advocate ignores suggestions that people die for all kinds of ideas that aren't true, for all types of realistic and plausible reasons.
Do you see why many atheists have an emotional reaction to proselytizing and insinuations against their collective characters?
Now, I'm not suggesting that people are right to treat you with hostility. There are theists here who are welcomed additions, who contribute in a robust and vigorous way to the discussions.
It's not that atheists usually have emotional reactions to the topic of theism. Many atheists I know don't even think about theism. I know I sure as heck don't in any ordinary day. What I and some other atheists have emotional reactions to is getting dragged into discussions about theism with people who clearly don't care about the discussion and who only want to walk away from the discussion after having told another person to fear the monster, tell us not to eat grapes, and to wait for the day when we'll see the green unicorn."