The Tenacity of Unreasonable Beliefs is a passionate yet analytical critique of Jewish, Christian, and Muslim scriptural fundamentalists. Schimmel examines the ways in which otherwise intelligent and educated Jews, Christians, and Muslims defend their belief in the divine authorship of the Bible or of the Koran, and other religious beliefs derived from those claims, against overwhelming evidence and argument to the contrary from science, scholarship, common sense, and rational analysis. He also examines the motives, fears, and anxieties of scriptural fundamentalists that induce them to cling so tenaciously to their unreasonable beliefs.

Schimmel begins with reflections on his own journey from commitment to Orthodox Judaism, through doubts about its theological dogmas and doctrines, to eventual denial of their truth. He follows this with an examination of theological and philosophical debates about the proper relationships between faith, reason, and revelation. Schimmel then devotes separate chapters to Jewish, Christian, and Muslim scriptural fundamentalism, noting their similarities and differences. He analyzes in depth the psychological and social reasons why people acquire, maintain, and protect unreasonable religious beliefs, and how they do so. Schimmel also discusses unethical and immoral consequences of scriptural fundamentalism, such as gender inequality, homophobia, lack of intellectual honesty, self-righteousness, intolerance, propagation of falsehood, and in some instances, the advocacy of violence and terrorism. He concludes with a discussion of why, when, and where it is appropriate to critique, challenge, and combat scriptural fundamentalists. The Tenacity of Unreasonable Beliefs is thoughtful and provocative, written to encourage self-reflection and self-criticism, and to stimulate and to enlighten all who are interested in the psychology of religion and in religious fundamentalism.