The True Believer: Thoughts On The Nature Of Mass Movements is a 1951 social psychology book by American writer Eric Hoffer, in which Hoffer discusses the psychological causes of fanaticism.
"Hoffer analyzes and attempts to explain the motives of the various types of personalities that give rise to mass movements; why and how mass movements start, progress and end; and the similarities between them, whether religious, political, radical or reactionary. He argues that even when their stated goals or values differ mass movements are interchangeable, that adherents will often flip from one movement to another, and that the motivations for mass movements are interchangeable. Thus, religious, nationalist and social movements, whether radical or reactionary, tend to attract the same type of followers, behave in the same way and use the same tactics and rhetorical tools. As examples, he often refers to Communism, Fascism, National Socialism, Christianity, Protestantism, and Islam."