We may think we know what defines religious fanaticism: violent action undertaken with dogmatic certainty. But the term “fanatic,” from the European Reformation to today, has never been a stable term. Then and now it has been reductively defined to justify state violence and to delegitimize alternative sources of authority. Unknowing Fanaticism rejects the simplified binary of fanatical religion and rational politics and turns to Renaissance literature to demonstrate that fanaticism was integral to how both modern politics and poetics developed, from the German Peasant Revolts of the 1520s to the English Civil War in the mid-seventeenth century. This book traces two entangled approaches to fanaticism in the long Reformation: the targeting of it as a political threat and the engagement with it as an epistemological and poetic problem. In the first, thinkers of modernity from Martin Luther to Thomas Hobbes and John Locke positioned themselves against fanaticism to dismiss dissent and abet theological and political control. In the second, the poets of fanaticism investigated the link between fanatical self-annihilation—the process by which one could become a vessel for divine violence—and the practices of writing poetry. Edmund Spenser, John Donne, and John Milton recognized in the fanatic’s claim to be a passive instrument of God their own incapacity to know and depict the origins of fanaticism. This crisis led these writers to experiment with poetic techniques that would allow them to address fanaticism’s tendency to unsettle the boundaries between reason and revelation, human will and divine agency.