This work investigates the historical phenomenon of religious enthusiasm, questioning how intense, subjective spiritual experiences have historically challenged established ecclesiastical structures. Ronald A. Knox, a distinguished Catholic theologian and Oxford chaplain, draws upon three decades of meticulous research to analyze the personalities and doctrines of various movements that prioritized direct divine inspiration over institutional authority. He constructs a framework that categorizes these movements, examining how their pursuit of spiritual purity often led to friction with the broader church hierarchy.

Scholars and theologians frequently cite this text as a foundational, albeit dense, study of the recurring patterns of religious fervor throughout history. Experts highlight the author's erudite prose and his ability to synthesize complex theological disputes into a coherent historical narrative.