John Locke's Letter Concerning Toleration is one of the most under appreciated texts in the liberal tradition of political philosophy. When read in conjunction with his Second Treatise, it clarifies the relationship Locke envisions between individuals and the Lockean state. The subject of the Letter is specifically religious toleration, but his general argument for toleration is also applicable to issues of more modern concern.

In the letter, Locke argues that all religious practices should be tolerated unless they are a threat to the proper functioning of the state. Some specific practices are not tolerated - Locke perceives the Catholic allegiance to the Pope, at that time, not only a religious leader, but also an influential foreign political leader, as a threat to the state, and he believes that atheists cannot be trusted by the state, since they have no higher power to whom they can swear an oath. Locke does not tolerate these individuals, because of his (inaccurate) perceptions of them, but religion is still not the basis for their non-toleration. (In the sense that others who are inherently untrustworthy, or bowed to a foreign ruler, would also not be tolerated, regardless of their religion).

The toleration of some other practices is situational. For instance, a state that normally has no law against individuals slaughtering animals (for food, et al) cannot prevent a religious sect from sacrificing an animal, but if that same state, needing meat for its troops in a time of war, bans all private citizens from killing animals, then this ban applies likewise to the sacrifice of animals as part of religious worship. This is not a state of license, in that the civil government does not actively promote a variety of (or for that matter, any) religious practices, but it is a state of negative liberty, in which the state remains neutral to the religious content of religious worship. Specific sects or acts of worship can be banned if they are "prejudicial to other men's rights" or they "break the public peace of societies," but they cannot be banned on religious grounds.

Some critics have argued that Locke's Letter is no longer very relevant: he deals only with religious toleration, and religious toleration is widely accepted and practiced in the modern Western world. However, the historical context of the Letter suggests it retains its relevance. In Locke's day, religion was not the dormant issue it is today; rather it was the most controversial issue of public debate. Before Locke, toleration was just something the underdog wished for in order to survive until he gained power over everyone else. Locke, however, goes beyond this pettiness and creates a theoretical defense of toleration as an extension of his political theory. While Locke probably did not imagine the controversial issues of political debate today, the broad basis for his defense of religious toleration implicitly justifies other sorts of social toleration in the modern world.

If a state is created for the purposes and by the methods Locke suggests in his Second Treatise, then the men who consent to form such a state retain a significant negative liberty of belief and action. Any of these beliefs or actions must be tolerated by the state unless they fail Locke's criteria for religious toleration, namely, unless they are "prejudicial to other men's rights" or they "break the public peace of societies."

This Routledge edition uses the original William Popple translation of Locke's Letter (which Locke published in Latin). Locke claimed that Popple undertook this translation without his permission, though the editors in this edition question the truth of this claim. In any case, the translation is at times more "radical" than Locke's original text. Horton and Mendus have also included a collection of essays written in response to Locke's letter that examine the relationship between the Letter and the Treatises and the modern relevance of Locke's argument for toleration. The need for toleration is as great in our own time as it was in John Locke's, and his contribution to the debate is likewise as valuable now as it was then.