In an introductory chapter, Harris calls for an end to respect and tolerance for the competing belief systems of religion, which he describes as being "all equally uncontaminated by evidence".

Insofar as religious belief fails to ground itself in empirical evidence, Harris likens religion to a form of mental illness which, he says, "allows otherwise normal human beings to reap the fruits of madness and consider them holy."

He contends that, far from being an aberration, the torture of heretics was a logical expression of Christian doctrine – one which, he says, was clearly justified by men such as Saint Augustine.

He also presents data from the Pew Research Center, purporting to show that significant percentages of Muslims worldwide would justify suicide bombing as a legitimate tactic.

[5][6] In an attack on what he terms "leftist unreason," Harris criticises Noam Chomsky among others for, in his view, displaying an illogical willingness to lay the entire blame for such attitudes upon U.S. foreign policy.

Harris also critiques the role of the Christian right in the United States, in influencing such areas as drug policies, embryonic stem cell research, and AIDS prevention in the developing world.

In what he sees as a steady drift towards theocracy, Harris strongly criticises leading figures from both the legislature and the judiciary for what he perceives as an unabashed failure to separate church and state in their various domains.

Next, Harris goes on to outline what he terms a "science of good and evil" – a rational approach to ethics, which he claims must necessarily be predicated upon questions of human happiness and suffering.

The only demons we must fear are those that lurk inside every human mind: ignorance, hatred, greed, and faith, which is surely the devil's masterpiece.Writing for The Independent, Johann Hari was largely positive, describing the book as "a brave, pugilistic attempt to demolish the walls that currently insulate religious people from criticism.

[11] In a review for Free Inquiry, the editor Thomas W. Flynn alleged that Harris had allowed his argument to become clouded by his personal politics and by his use of spiritual language.