John Locke believed that the magistrate should not punish sin but only violations of natural rights and public peace (1689)
Found in The Works, vol. 5 Four Letters concerning Toleration
John Locke (1632-1704), in his Letter on Toleration, argued that sins should not be punished by the magistrate. Only acts which are "prejudicial to other men’s rights" should be legally punished:
… it does not follow, that because it is a sin it ought therefore to be punished by the magistrate. For it does not belong unto the magistrate to make use of his sword in punishing every thing, indifferently, that he takes to be a sin against God. Covetousness, uncharitableness, idleness, and many other things are sins, by the consent of all men, which yet no man ever said were to be punished by the magistrate. The reason is, because they are not prejudicial to other men’s rights, nor do they break the public peace of societies.
Living in the 17th century Locke had seen or had heard about the terrible things which had been done in the name of religion as Christian Europe divided into Catholic and Protestant sections and fought to the death over their scriptural differences. Locke was a middle of the road supporter of toleration (he denied it to atheists and Muslims for example) but considerably advanced compared to some of his contemporaries. In this passage he clearly states that, unless another person’s rights are violated (such as their property or liberty) the magistrate has no right under law to punish a person for the very nebulous and disputed concept of “sin”. Note the related works of Pierre Bayle and Voltaire on this topic.