This book argues that liberty of conscience remains a crucial freedom worth protecting, because safeguarding it prevents political, social, and psychological threats to freedom. Influential early modern theorists of toleration, John Milton, Thomas Hobbes, Baruch Spinoza, and Pierre Bayle, I show, defend liberty of conscience by stressing the unanticipated repercussions of conformity. By recovering the intellectual origins of liberty of conscience in early modern politics and situating influential theorists of toleration in overlooked historical debates on religious dissimulation and hypocritical conformity, I demonstrate that infringements on conscience risk impeding political engagement, eroding civic trust, and inciting religious fanaticism. While this is a book about freedom, it is also a book about threats to freedom, specifically conformity, hypocrisy, and persecution. It considers the social, psychological, and political harms done by political refusals to tolerate religious differences and allow individuals to practice their religion freely in accordance with the dictates of conscience. By returning to a historical context in which liberty of conscience was not granted to religious dissenters –but rather actively denied – this book foregrounds Bayle’s argument that coercing conscience exacerbates religious fervor and inflicts significant psychological harm on dissenters, thereby undermining the goal of cultivating social cohesion in politics. In controversies on the politics of conscience, I suggest that we acknowledge that refusals to tolerate claims of conscience – while perhaps well-grounded in democratic laws and norms – might exacerbate conscientious fervor and empower resentment against the state. This Baylean intuition does not necessarily tell us where to draw the limits of toleration – what should be tolerated and what goes beyond the pale – but it does tell us something about how to approach invocations of conscience and what to expect when we deem something intolerable.