Jihadi ideologues mobilize Muslims, especially young Muslims, through an individualist-centered interpretation of Islam. Appealing to a classical defense doctrine, they argue that the mandates of jihad are the individual duty of every Muslim and therefore transcend and undermine both the authority of the state and the power of parental control. Yet emphasizing the duty and right of individually initiated jihad is only one side of do-it-yourself Islam. The other involves protecting the purity of doctrinal beliefs against deviation, even by fellow jihadis. The pursuit of doctrinal purity has led some jihadis to resort to takfir, a pronouncement that declares fellow Muslims unbelievers and makes it legal to shed their blood. Set against the background of the Kharijites, Islam’s first counterestablishment movement, this book explores the religious philosophy underlying jihadism, arguing that because the Kharijites’ idealistic and individualistic ideology forces members to deploy takfir against one another, they are in fact hastening their extinction as a group.