[Update: Cat actually likes it, so I'm going to turn it loose. Here goes...]
In a related response to Cat's remarks about her own tolerance, I wrote:
I don't think there is any "should." It is up to you. Tolerance, I think, has to include some idea of not appealing to some universal "should."If we are sure of universal ethics, if we believe we know what they are, and can state them in simple declarative language, it is from there a short step to the idea that one may treat that ethics as a law book, as the word of "god" or "gods," and decide who or what is in accord with those ethics, and who one may praise, and who, curse.
No matter how carefully we weave our net of laws, something always slips through. No law book substitutes, in the end, for human reason and judgement.
I believe (which means, I cannot prove to everyone's satisfaction) that there are people who feel called to serve a "higher" purpose. Christians talk about "vocations" (literally "callings"), but I know Jews and neo-pagans who speak in similar terms, and perhaps one Muslim woman. One neo-pagan priest I know has spoken about trying to assemble "druidic" rituals as a high school student, long before he had any coherent philosophy or theology to base them in. The calling, even, need not be to what is usually called a religion or philosophy—I know people who treat abstract ideals like peace and justice in that way, or a scientific or academic discipline. Even, perhaps, atheism.
People who hear the call often believe it comes from some source of universal truth. But how can it? The callings often contradict each other: a god who calls people to atheism would be a very strange god. Rather, I believe (which means, remember, I cannot prove to everyone's satisfaction) people choose their higher truths. Regardless of how important it is to the believer, it is still their belief. The alternative to the idea that higher truths are humanly chosen is the belief that some number (usually one) of them are truly the word of god and all the others are false, evil, to be stamped out.
How are we to respond to this belief, the philosophical form of religious intolerance? Why respond at all? Belief is after all a personal choice. Yet there are reasons both personal and social. On the personal level, who wants to be proselytized? And the most aggressive proselytizers are invariably the ones who will push their causes regardless of need, and sometimes to the harm of the people they persuade. Likewise, the most aggressive believers are willing to harm non-believers for their non-belief. So there is a matter of self-defense. At the social level, conflicts between beliefs are the most enduring and destructive, because they go right to the core of identity, and can never be resolved. People fight and die for their gods. Unless we want a society in which is constantly at war with itself, some customary and, perhaps, legislated form of tolerance is a social necessity.
What do we say to the believers? We say, I think, "Is this how you serve your belief?" For there are very few higher ideals which demand the abuse of others. There is a downhill slope in conversion: a slide from teaching and offering the truth to pressuring, and ultimately to violence. As the slope is trodden, the believer moves more and more from teaching their ideal to using raw power over their subjects, and the seductions of power overcome all else. The ideal is lost.
Equally, I think it best to ask ourselves to be tolerant. If we claim skepticism, we cannot ethically force our beliefs on others. "There is no compulsion in religion," as the Qu'ran says.
At the social level, I think, one of the best tools of tolerance has turned out to be secularism. Societies, in the end, are devices for living in the world. One does not need to know the purpose of life to do that. The attempt to order a mass society (as opposed to a monastery) around a religious unity is doomed. There are always schisms, heresies, ..., conflict. The United States has mandated tolerance in its first amendment; many of the original colonists were religious radicals and historical religious conflicts were fresh in the minds of the Framers. Historically, this has been one of the most successful of freedoms of the Bill of Rights. While the US's English Protestant majority has been powerful, their ability to suppress other faiths was limited and, as a result, the United States was spared much pointless conflict. Our religious reactionaries, who would set up a theocracy, forget how badly theocracies fare in history. They fail of their ideals, almost as soon as they gain political power and power becomes an end in itself.
So, to recapitulate the argument, religious intolerance as a philosophy derives from the idea that, somehow, ethics can be reduced to a rule book. The desire of some people to serve a higher purpose leads to the conclusion that their chosen purpose is the only valid one, and all others are false and to be suppressed. Religious tolerance is personally desirable and socially necessary: socially necessary because without it society is at risk of repeated civil wars, personally desirable because intolerance leads to abuse and, ironically, the failure of religious ideals. There are compelling reasons to be personally tolerant and to embrace religious tolerance as a secular ideal.