What are the relationships that should exist between the State and the Church, or rather between the State and the Churches? To answer this question, the Letter on Tolerance takes the path of a double reflection on the nature of belief and power. Belief does not depend on will, while power, which is essentially coercive, can only influence the conduct of men in actions where they have the possibility of modifying their conduct by a voluntary decision; however, since belief does not fall into this category, it is not one of the possible objects of political authority. At the heart of the reasoning which leads to the idea of ​​tolerance is therefore a very simple thesis: our thoughts are not in our power. Combined with the conviction that, in religious matters, only sincerity saves, it constitutes the foundation of the Lockean conception of tolerance.