BeliefsWe all have countless beliefs in things. It's a necessary part of making sense of the world. Whether it's the belief in gravity, in the equality of men and women, or that unicorns exist. Beliefs can be founded on two things: evidence or faith. In practice, since no belief can be 100% proven based on evidence, we tailor the strength of different beliefs based on the strength of evidence. There are many things we may believe that we haven't explicitly gone out and searched for evidence for, and it might be tempting to call this faith, but it really isn't, and always has a basis in facts and evidence.
For example, you might say that I have faith that Italy exists, since I have never been there and I'm trusting the word of others who say it exists. Is this faith? Not at all. There is a wealth of different types of data, such as books, movies, documentaries, conversations with people, that are on the whole consistent in their claims to Italy's existence. But what is more important, there are plenty of ways I could go out and gather further evidence on whether Italy exists or not if I so choose. No one is forcing me to just take their word on the issue.
FaithThe key factor that makes religion dangerous is faith. Faith, far from being a virtue, is what makes people cling to bad ideas despite lack of evidence, or even worse, despite direct evidence that the belief is false. You can't reason with faith. You can't present evidence to shake it. People with strong beliefs based on faith are often proud of the fact that they do not require evidence for their belief, and religions sometimes even tap into this and promote it as virtuous.
Faith isn't unique to religions, though. It is a typical part of many types of ideology. When you get a set of beliefs that must be accepted without proof, you start getting into dangerous territory. And when you suppress debate, discussion and contrary viewpoints to that ideology, that's when evil things tend to happen. This is the cause of a lot of the large scale suffering that does not stem from religion, e.g. Soviet Communism, pure unregulated capitalism.
The Reality CheckEvidence-based beliefs naturally are subjected to reality checks on a regular basis. Every piece of evidence is a test of whether a belief is consistent with reality. It can sometimes take a long time for the truth to be determined, but at least a mechanism exists for this to happen, and so we can be confident that bad beliefs will eventually be revealed as more evidence is gathered. (Note that this confidence is not faith, since it is based on a strong provable history of this mechanism working, and without a solid reason to expect this to change, the most defensible position is to expect the future to follow the same pattern as the past).
Faith-based beliefs don't typically rely on evidence and so don't have reality checks built in, but as long as a belief makes claims about the real world, evidence can be used to strengthen or weaken it. So, for example, if a person believes on faith that the world is 6000 years old, there is plenty of evidence that can be raised against this belief. The believer may choose to ignore the evidence and keep believing anyway, but the burden of cognitive dissonance will grow stronger as the evidence piles up, and there is at least a chance that eventually the believer will be forced to deal with it.
But what about beliefs that don't make claims about the real world? What if you believe that your suffering in this world will be rewarded after you die? What if you believe that killing a bunch of innocent people is what God wants you to do, and he will give you virgins in the afterlife if you do it? What if you believe that a person must not end their suffering from a horrible terminal illness by taking their own life, or having another person assist with this, because there will be worse punishment after death for it? What if you believe that people reincarnate and disabled/deformed people did something wrong in their previous life, and so deserve their suffering in this life rather than our support and kindness?
The danger with these sorts of beliefs is that they can never be disproved. There is no possible evidence, even in theory, that can prove the beliefs to be false. There can never be a reality check. When religions make claims about the physical world, such as the age of the universe, or that a piece of wafer gets converted into the flesh of a dead man, these claims can be made to smash hard against the rocky shore of reality. But when claims are made that fall beyond the realms of the physical universe, such as the nature of god and his decree of what is right and wrong, these beliefs float, untouchable, above the messy battlefields of reality where beliefs survive or die based on evidence.
Final ThoughtsReligion deserves to be singled out as a uniquely dangerous faith-based belief system, unlike other faith-based ideologies such as political, social, and economic systems, or straight ignorant beliefs that simply ignore evidence, such as sexism and racism, because it makes claims that can never be checked against reality, not even in principle. When these claims are used by believers to justify behaviours in the real world that negatively affect other living beings, it is very hard to bring about voluntary change from believers, because there is no way to prove to them that their core beliefs are wrong. Only by demanding intellectual honesty and getting believers to admit that their beliefs have no proof, and that believing something simply because you want it to be true is not good enough, can you bring about change, and this is far from a trivial task.
Acknowledgements: This post was inspired by chapter 3 of the book Why Are You Atheists So Angry? by Greta Christina, a very good book based on the atheist writings in her blog. I highly recommend the audio version of the book, enthusiastically read by the author herself.