The concept of evolution is widely considered to be a foundational building block in atheist thought. Leaders of the New Atheist movement have taken Darwin's work and used it to diminish the authority of religious institutions and belief systems. But they have also embraced it as a metaphor for the gradual replacement of religious faith with secular reason. They have posed as harbingers of human progress, claiming the moral high ground, and rejecting with intolerance any message that challenges the hegemony of science and reason. Religion, according to the New Atheists, should be relegated to the Dark Ages of superstition and senseless violence.

Yet Darwin did not see evolution as a linear progression to an improved state of being. The more antagonistic members of the New Atheist movement who embrace this idea are not only employing bad history, but also the kind of rigid, black-and-white thinking they excoriate in their religious opponents. Indeed, Stephen LeDrew argues, militant atheists have more in common with religious fundamentalists than they would care to admit, advancing what LeDrew calls secular fundamentalism. In reaction to fundamentalist Christianity and Islamism, this strain of atheism has become an offshoot of the religion it tries so hard to malign.

The Evolution of Atheism outlines the essential political tension at the heart of the atheist movement. The New Atheism, LeDrew shows, is part of a tradition of atheist thought and activism that promotes individualism and scientific authority, which puts it at odds with atheist groups that are motivated by humanistic ethics and social justice. LeDrew draws on public relations campaigns, publications, podcasts, and in-depth interviews to explore the belief systems, internal logics, and self-contradictions of the people who consider themselves to be atheists. He argues that evolving understandings of what atheism means, and how it should be put into action, are threatening to irrevocably fragment the movement.