John Locke's "Letter concerning toleration" is one of the canonical English-language texts in the history of the idea of toleration. Its publication in 1689 sparked a heated debate with Anglican cleric Jonas Proast. This text presents an examination of John Locke's arguments in defence of "A letter concerning toleration" and of their significance for modern political thought. The author considers the Locke-Proast controversy from the standpoint of political theory, examining Locke's and Proast's texts and tracing their relationship to later discussions of toleration. He reconstructs the grounds of the dispute, drawing attention to the long-term importance of the arguments and evaluating their relative strength. He then examines issues of toleration in later contexts, specifically James Fitzjames Stephen's critique of John Stuart Mill, the perfectionist alternative to contractualist liberalism, and the view that the traditional attachment to toleration must, by the force of its own arguments, move from liberalism to a defence of a much stronger form of democracy. Arguing that the exchange between Locke and Proast marks a turning point in the intellectual history that has helped to structure the terms of modern political debate, this work presents a case for thinking that the exchange between Locke and Proast is as important for the 20th century as it was for the 17th.