In the Feyerabend lectures on Natural Law, Kant addresses the topic of freedom of religion and thought in his commentary on the title "The Right Regarding Religion and the Church” of Achenwall’s Natural Law. Kant goes beyond the discussion proposed by the jurist and introduces two central ideas to his conception of Enlightenment, which will be developed in “An Answer to the Question: What is Enlightenment?”: the idea of self-legislation of the people and the distinction between private and public uses of reason. In this paper, I will first compare Kant’s and Achenwall’s views on freedom of religion and conscience and then show how the idea of self-legislation leads Kant to establish clear limits to the sovereign regarding matters of religion. Then, I will argue that the development of Kant’s idea of public use of reason results from the historical debate about the meaning of the concept of Enlightenment that took place between 1783 and 1784, especially regarding Ernst Klein’s assertions on freedom of opinion and freedom of the press.