This book provides an overview of the theological views on takfīr in classical and medieval Islamic thought, explores the Islamic context of the concept, and considers the following questions: In what context does the term appear in the Qur'ān and ḥadīths (the sayings of the Prophet Muḥammad)? In what circumstances did Muslim theologians engage in takfīr, and what were the social, religious, theological, and political implications on the society? How did the meaning of takfīr evolve in classical and later Islamic theology, and what justifications did prominent medieval Muslim theologians and jurists provide for declaring other Muslims kuffār? This book attempts to demonstrate how takfīr has evolved throughout the centuries from being a term used to condemn (and warn against) unacceptable actions to a term used on individuals and communities to strip them of their belief, dignity, and linkage to the Divine (i.e., declaring them unbelievers). This book also attempts to shed the light on how the theological discourse on takfīr in classical and medieval Islam has made its impact on twentieth- and twenty-first century groups.