The problem with some delusions is that they are so rife, ubiquitous, politically correct, taken so matter-of-factly by civilization, and even legitimized and legalized (to the extent that some societies penalize those who don't subscribe to them), that it is only with difficulty and effort that we can prevent ourselves from being overwhelmed by them and indoctrinated and brainwashed. Of course I am talking about none other than the delusion known as supernaturalism.
Doctoral candidate in neuroscience Sam Harris has some thoughts about this phenomenon, about delusions that survive to our day even as we're well into the 21st century.
Faith and Madness
by Sam Harris
Is a person really free to believe a proposition for which he has no evidence? No. Evidence (whether sensory or logical) is the only thing that suggests that a given belief is really about the world in the first place. We have names for people who have many beliefs for which there is no rational justification. When their beliefs are extremely common we call them "religious"; otherwise, they are likely to be called "mad," "psychotic," or "delusional." Most people of faith are perfectly sane, of course, even those who commit atrocities on account of their beliefs. But what is the difference between a man who believes that God will reward him with seventy-two virgins if he kills a score of Jewish teenagers, and one who believes that creatures from Alpha Centauri are beaming him messages of world peace through his hair dryer? There is a difference, to be sure, but it is not one that places religious faith in a flattering light.
It takes a certain kind of person to believe what no one else believes. To be ruled by ideas for which you have no evidence (and which therefore cannot be justified in conversation with other human beings) is generally a sign that something is seriously wrong with your mind. Clearly, there is sanity in numbers. And yet, it is merely an accident of history that it is considered normal in our society to believe that the Creator of the universe can hear your thoughts, while it is demonstrative of mental illness to believe that he is communicating with you by having the rain tap in Morse code on your bedroom window. And so, while religious people are not generally mad, their core beliefs absolutely are. This is not surprising, since most religions have merely canonized a few products of ancient ignorance and derangement and passed them down to us as though they were primordial truths. This leaves billions of us believing what no sane person could believe on his own. In fact, it is difficult to imagine a set of beliefs more suggestive of mental illness than those that lie at the heart of many of our religious traditions. Consider one of the cornerstones of the Catholic faith:I likewise profess that in the Mass a true, proper, and propitiatory sacrifice is offered to God on behalf of the living and the dead, and that the Body and the Blood, together with the soul and the divinity, of our Lord Jesus Christ is truly, really, and substantially present in the most holy sacrament of the Eucharist, and there is a change of the whole substance of the bread into the Body, and of the whole substance of the wine into Blood; and this change the Catholic mass calls transubstantiation. I also profess that the whole and entire Christ and a true sacrament is received under each separate species.Jesus Christ--who, as it turns out, was born of a virgin, cheated death, and rose bodily into the heavens--can now be eaten in the form of a cracker. A few Latin words spoken over your favorite Burgundy, and you can drink his blood as well. Is there any doubt that a lone subscriber to these beliefs would be considered mad? Rather, is there doubt that he would be mad? The danger of religious faith is that it allows otherwise normal human beings to reap the fruits of madness and consider them holy. Because each new generation of children is taught that religious propositions need not be justified in the way that all other must, civilization is still besieged by the armies of the preposterous. We are, even now, killing ourselves over ancient literature. Who would have thought that something so tragically absurd would be possible?
(Sam Harris, The End of Faith, New York: W.W. Norton, 2005, p. 71-73)