"Why Question Beliefs? Dangers of Placing Ideas Beyond Doubt, and Advantages of Freethought" -by Vexen Crabtree (2009): 

1) Crazy Beliefs are Dangerous 

1.1. Beliefs Have Consequences

Crazy beliefs are not harmless. Although many daft superstitions are obviously counterproductive, some of them pass into general culture and cause entire nations to behave irrationally on certain topics1. Take, for example, an incident in 2015 that saw a mob of 1,000 Hindus attack a small family of Muslims in India: A rumour had broken out that a cow had been slaughtered. Vigilantes from Save the Cow prompted a mob to appear on site, and proceeded to, amongst themselves, blame a nearby Muslim family (no slaughtered cow was found). They appeared at the house, where the family were sleeping, and beat the husband to death and left his boy in critical condition in hospital. The press got involved and Save the Cow explained their religious duty as Hindus to protect cows, which are sacred. A local politician from the Bharatiya Janata Party, Lakshmikant Bajpayee, defended the mob saying that there had a been a failure of local police to respond to the rumour adequately2. The issues are (1) that the slaughter of a cow - even if it had actually happened - is none of the business of local Hindus. It doesn't matter that they consider it sacred - other people do not. And (2), they should not be trying to force others to follow their own superstitions. Likewise, politicians should not be encouraging them - they should be representing all citizens including those with non-Hindu beliefs. Entire communities and cultures are being negatively affected by religious nonsense.
On a larger scale than mere mob violent, take a theory which started out as amateur science, and proceeded to win approval at a national level despite the protests of the scientific community: The theory of a 'hierarchy' of delineated human races, with Nordics at the top and Negros at the bottom (with a special place reserved for the Christ-rejecting Jews). It combined pseudoscientific anthropology with religious prejudice to create history's most horrific apparatus of prejudiced murder. The Nazi's ideology of race and euthanasia derived from erroneous beliefs based on faulty theory; and it blossomed into a wave of terror. It is no exaggeration to say that, in a world where nuclear weaponry is becoming more easily available, that beliefs need to be questioned and challengedcontinuously and vigorously. 

Beliefs can spread, and sometimes while they may start out as something merely odd, they can cause atrocities when they become norms or when they are used to justify violence. For example the 1857 Mutiny of the Cepoys was started as a "revolt byIndian soldiers against the introduction of cartridges greased with pig- and cow-made grease" amongst other superstitious and religious objections4. This is taken up as a major theme of "The End of Faith: Religion, Terror and the Future of Reason" by Sam Harris (2006):

"Even apparently innocuous beliefs, when unjustified, can lead to intolerable consequences. Many Muslims, for instance, are convinced that God takes an active interest in women's clothing. While it may seem harmless enough, the amount of suffering that this incredible idea has caused is astonishing. The rioting in Nigeria over 2002 Miss World Pageant claimed over two hundred lives; innocent men and women were butchered with machetes or burned alive simply to keep that troubled place free of women in bikinis. Earlier in the year, the religious police in Mecca prevented paramedics and firefighters from rescuing scores of teenage girls trapped in a burning building. Why? Because the girls were not wearing the traditional head covering that Koranic law requires. Fourteen girls died in the fire; fifty were injured. Should Muslims really be free to believe that the Creator of the universe is concerned about hemlines?”

Take Mohammed Merah, a French national originally from Algeria, whose story was told by The Economist (2012)6. His beliefs led him to the wild conclusion that in order to be rewarded by God and thusly to live forever in paradise, he had to conduct a series of terror attacks in France. He did so, killing four adults and three children. He did it because in France, no-one is allowed to wear complete face and body coverings in public. But his victims included members of an Israeli family who were not even French and therefore were completely unrelated to the ban on complete body coverings - Mohammed Merah's excuse was that he also opposed Israelbecause it was also an enemy of Islam. When they raided his house on 2014 Mar 22, he shot and killed three paratroopers, before being himself killed. Try to imagine, exactly, how it is that this immoral monster can think that God endorses his actions? Because he has very strange beliefs, but not only that, but that he believes he must act on those beliefs no matter what. Only religion can instil such a dramatic sense of ultimate urgency and divine necessity upon murderers.
It is not just the worldwide monotheistic religions that provide motivation for irrational violence, sectarianism and murder. Superstitious beliefs in general can often provide people with the ultimate excuse for acting against those who they simply don't like. Take the example of witchcraft, widely believed in across the world except in the largely secular countries of the developed world. Here's a case from Assam, in north-east India.

“In early 2007 Mrs [Ranjita] Basumatary was driven from her original village after her neighbours accused her of being a dain - a witch. Around 100 villagers surrounded her home and beat her with sticks, leaving her badly bloodied and bruised. After receiving death threats, she fled with her husband and three children. [...] Local jealousy seems to have prompted the accusations of witchcraft. Her family had prospered, leasing livestock to other villagers. It led to resentment. When children in the village fell sick, the ojha accused Mrs Basumatary of casting spells - his own charms and potions having failed. Her case is not an isolated one. At least 17 people were killed in witch-hunts in the area last year.”
The Economist (2012)

If only the locals understood the various neurological, physical and subtle causes of superstitious beliefs that mislead us humans into such superstitions. I lay out these causes in Errors in Thinking: Cognitive Errors, Wishful Thinking and Sacred Truths and The False and Conflicting Experiences of Mankind: How Other Peoples' Experience Contradict Our Own Beliefs. If only the locals understood the benefits of opening all beliefs to critical analysis. Their absurd behaviour can be fixed, if only they learn to question their own beliefs and therefore adopt a more rational outlook. But they can't, because religious taboos and the authority of the local spiritual leaders prevent it, just as similar people have done across the world, substituting dogmas for sensible inquiry into the world, and ousting those who dare to think differently.

Although beliefs might start off as fads, rumours, stories and the like, they can balloon out of control. A certain critical mass is reached when the populace as a whole no longer question the belief and it is at this point that it becomes a dangerous piece of dogma: unquestioned. All beliefs should be questioned continually, for fear that they will become monsters, blinding people not only to greater truths, but to moral truths as well.

1.2. The Belief that a Leader Can Discern God's Will is Very Dangerous

What dangerous cults, wayward doctrine and abusive leaders all have in common is that their followers have accepted that those in charge can discern God's Will - or the will of fate, the gods, the universe, whatever - and that they have the authority therefore to override normal rules of behaviour. So, leaders in cults are often sexually promiscuous because they have special permission from god, they can instruct followers who to kill, and what social and legal rules their adherents can ignore. For example Charles Manson's followers claimed that Manson was above the because he was divinely guided8, Jim Jones' instructions caused the death of 800 of his followers in a suicide pact, and during assaults crusaders were told to kill everyone - Christian, Muslim and pagan, and "let God sort them out" once they were dead, rather than attempt to tell who were local Christians. In all cases, the main cause of the problem was that the ordinary followers did not have the skepticism to ask if those in charge were really capable of divining the will of God, or if religious beliefs should really override proper morality.

1.3. Faith: Why Religious Beliefs Are so Vehemently Defended

"Even standard sociological inquiries about beliefs and the history of beliefs can be found offensive simply because the attempt to rationally describe belief requires questions to be asked about how beliefs work."

“The sociologist of religion may also offend a person's religious sensibilities by subjecting his or her beliefs to rational scrutiny.”
"Gods in the Global Village"
Lester R. Kurtz (2007)   

Atheists and scientists do not kill each other over their beliefs. The adherents of superstring theory have never killed opposing theorists, and Lamarckian Evolutionists never killed any Darwinian Evolutionists on account of their beliefs. Newton and Einstein may have disagreed, but they refrained from violently attacking each other's followers. Like them, Arius and Athanasius disagreed over theory in the 4th century, although in their case it wasn't physics, but about the nature of Christ. The Arians and the Nicene Christians, however, soon ended up damning each other to hell because of the other's "wrong" beliefs, and then resorted to murder, aggression and burning until the Arians had been wiped out. Well, that is one way to settle a theoretical dispute. But why is it thereligious way? There is something about religious beliefs that leads to violent intolerance. I think it is this: the beliefs that you cherish, but which you think are maybe silly or untrue, are the beliefs that you will defend most irrationally and most aggressively. It's a defence mechanism. Rather than subject dodgy beliefs to the rigors of debate and questioning, it is easier to claim outrage and act aggressively when dodgy beliefs are challenged. This is why scientists, who want to learn which theories best describe the truth, actively engage in debate without ever, in history, killing each other over their differences with other strands of scientists.

When it comes to disputes about the world, political and cultural differences between groups can often be dealt with democratically, through argumentation and debate, with both parties trying to convince the others that they are right. It continues because each side thinks it is possible to conclude the dispute through discussion. Compromise keeps things from breaking down: you give a little in one area, but have to give up in another. But religionists can come to deny any chance of compromise. Those with stern religious beliefs often believe various issues have a universal, absolute and cosmic significance. They will not compromise on their position. Malise Ruthven in his book on fundamentalism warns that this is particularly dangerous10. It is the basis for fundamentalism. Religious differences often become violent, endless struggles, because both sides elevate their struggles to ones not between them and us, but between good and evil itself. By giving arguments a cosmic, absolute and universal significance, religious groups make violent solution the only recourse. The battles between Israel and its neighbours is a case in point.

A commentator said to me that it would do no good to eradicate religion. He said "then they'd just kill each other in the name of something else. Like which football team they support!". I still think we would be better off. Football teams do not claim to be divinely inspired. They do not force upon people any particular intellectual framework, nor link it to moral theory. Under footballism, people are still free to enquire about the world with a free mind. The fact that religions claim divinity, that they claim absolute truth and link morality, society, authority and philosophy all into one whole, makes people more likely to fight and die for them. What is so weak aboutreligious truths that they require defending with such bloodshed? It is this: People would rather cling to wishful thinking and false hopes, rather than face the complex realities of life. Simple answers appeal to people more than complicated scientific ones.

Because religious people secretly doubt religious beliefs, they do not permit them to be calmly questioned. They fear that their beliefs will unravel. Instead, they declare that faith is greater than intellectualism (in other words: they want to continue believing even though the evidence is against them). They declare that it is offensive to question their beliefs. They declare that questions are wrong! And if you persist in your questioning as a person, they'll declare you an intolerant bigot. If two such groups of faithful people meet, the consequences are dangerous for all in their midst. The starting point of this slippery slope was when individuals ceased to allow their beliefs to be calmly debated and questioned.

1.4. Suicide Cults

“In 1978 over 900 people died when the People's Temple (frequently known as Jonestown) murdered their (276) own children with poison. The rest of the community then followed suit, killing themselves (and shooting some). They had previously practised the suicide routine. Their leader shot himself. He was American Rev. James (Jim) Warren Jones, an ordained priest in the mainstream Christian Church (Disciples of Christ).”"Religion, Violence, Crime and Mass Suicide: The People's Temple" by Vexen Crabtree (2009)

This is clearly a case of some 900 people having some very odd ideas about what actions are permissible and why. Their leader had followers who testified that he could cure cancers and disease with his hands. If members instinctively thought more critically, skeptically, and viewed reality through more scientific eyes (understanding the placebo effect, for example), such beliefs could not have been sustained. The People's Temple is by far not the only suicide cult to have some very odd beliefs.
“Another American group, the Branch Davidians, [...] started out with Biblical ideas about the cataclysms of judgement day, and ended up stockpiling weapons. It culminated with the Waco siege where over 80 of the religionists died during a shoot-out with authorities in 1993.

Irrationality and susceptibility to believe some unlikely things about the universe can lead to ideals and sectarianism that separate 'others' from their humanity, and allow despicable acts to be undertaken. Aum Shinrikyo was the religious movement responsible for the 1995 sarin gas attack on Tokyo's subway that killed a dozen people and injured thousands. The movement had also already murdered others in order to protect itself. The leader believed in karma, and preached that murder was justified because it stopped people accumulating bad karma. He had picked up Christian ideas, and preached that such actions were an act of mercy, and started preaching about Armageddon.”
"Religion, Violence, Crime and Mass Suicide: Apocalyptic Survivalism" by Vexen Crabtree (2009)

It is a similar story with the Order of the Solar Temple, Heaven's Gate, and others. All groups start out with some beliefs that are acceptable to many, but, the system of beliefs becomes built up, with idea on idea, until the entire group are completely impossible to understand. It is always a slippery slope, and at each stage, there are not enough people to stand up and question the validity of the principles behind the beliefs, the source of the experiences/revelations, and the possibility of mass delusion. People invest so much in these beliefs, defending them from outsiders, that they become more important than life itself.

This is why questioning beliefs is not something that should be restricted to "other people's beliefs". We should question our own beliefs. Our entire community of friends could be making assumptions about reality that are unfounded. Entire groups of people can misinterpret phenomenon and experiences, simply because they are unacquainted with the ways in which our thoughts can deceive us, and of how to approach reality in a scientifically-minded way. Questioning beliefs is not something that should be restricted to "other people's beliefs". We should question our own beliefs.

1.5. Religious Fundamentalism

It is easy to see how the acceptance of ideas and the interpretation of personal experiences without taking due heed for the way our brains can trick us, is a combination that can lead communities down paths away from normal society. Some groups such as the ones already discussed on this page become suicide cults, others remain as fundamentalist cells within mainstream religions, or sometimes become religious groups in their own right. It is dangerous when their beliefs become seen as unquestionable because they happen to be part of a religious worldview. Sam Harris in his book against religious fundamentalism and extremism (2006) warns that when we place someone's opinions beyond criticism because they are sacred to them, we place that person beyond rehabilitation to common sense.

Not long after the arise of Christianity, the Church Fathers argued that The Bible contains everything we need to know. This doctrine was deadly poison to science and to human development, and it found its greatest expression at a continental level during the European Dark Ages. The theologian Robert M. Price warns that even today many people use the Bible as a tool to make others bypass rational thought, often by "cultivating superstitious fears", in order to spiritually strong-arm others around to their own stern point of view.

“The authority of the Fathers, and the prevailing belief that the Scriptures contain the sum, of all knowledge, discouraged any investigation of Nature. If by chance a passing interest was taken in some astronomical question, it was at once settled by a reference to such authorities as the writings of Augustine or Lactantius, not by an appeal to the phenomena of the heavens. So great was the preference given to sacred over profane learning that Christianity had been in existence fifteen hundred years, and had not produced a single astronomer.”
"History of the Conflict Between Religion and Science" by John William Draper (1881)12

Harris argues that the blame doesn't only lay with the fundamentalists themselves, but with the majority of non-extremists who sit in the middle ground, facilitating an environment where crazy beliefs can foster without question. In all communities that place large parts of their mythology into a "holy", "sacred", "unquestionable" or "god-given" category, a large space is created for more extreme beliefs to take hold in the same categories, and lead the community down some potentially dangerous paths. The way to end all this is to put an end to the idea that religious beliefs should not be routinely questioned.

1.6. Confronting Taboos and Blasphemy

"It harms everyone. A social taboo isn't just something that isn't voiced, it nearly always becomes something that is unquestioned and unchallenged. It causes mental stagnation and confusion. Taboos and unchallenged dogmatic statements influence our actions in mostly negative ways.

I believe wholeheartedly that life is a never-ending search for understanding and social taboos and ignorance are two of our hindrances. They are both fuelled by stupidity!”

“Lack of doubt can appear in people who become confident they're on the right track, etc. It breeds stupidity and complacency, leading to what amounts to counterproductive pride, where they end up defending an erroneous stance. A strong emphasis on continual doubt and personal development will hopefully reduce stupidity.” 

“In the name of making people think and smashing stereotypes and assumptions.You shall think!! And if you cannot break free from the superstitious and paranoid things that society tells you about religious symbols, you will forever struggle!”

“Blasphemy is required to weed out people who would restrict our speech, not for fear of us insulting people, but for us questioning concepts. The point is to make people realize how absurd the concept of blasphemy is.”

  by Vexen Crabtree (2009):