Perhaps the worst mistake in human history was made by those unknown people, lost in the dark backward of pre-literate time, who first decided to take tall tales literally, thus beginning the Religious Error. Contra that good atheist Gore Vidal, it's not monotheism that's our species' greatest mistake. The more fundamental error (in the etymo-scatological sense of 'issuing from the fundament') is the reification of an imaginary 'spiritual' realm into that vast pseudodoxia epidemica we call religion and spirituality. The first human beings who understood that they could control other human beings by deploying certain carefully selected fictions (which the deployers themselves might have considered factual) set our species upon the path that has led to murder, genocide, war, and the meaningless deaths of millions. But for the intellectual laziness and/or enforced ignorance of the vast majority of the human race--a situation favorable to arbitrary power since time immemorial--we would have outgrown the gods millennia ago. At the very least, given the vast advances made by science since the Enlightenment, there is neither need nor excuse for religious belief today. It is indeed the opium of the people (tragically redundant in an age of Oxycontin), and it has a similar side effect: wastage of life.
The only serious question I have about religion (and spiritual belief generally) is whether it is best analogized to opium (Marx) or a virus (Dawkins). The wasting drug or the wasting disease?
Philip Roth once said in an interview that he was the least religious person he knew. He could say that only because he never met me.