In A Letter Concerning Toleration, John Locke addresses the attitude of the state towards religious sects. The essential problem, as Locke sees it, is how to prevent religious differences from causing civil unrest. Earlier thinkers, including Thomas Hobbes, had argued that the state should impose a single religion on everyone to prevent such differences. Locke views this as futile, believing that such an imposition is far more likely to cause rebellion than the quell it.

Instead, Locke addresses the problems of religious discord by proposing a compromise along the same line as Christ's command to "render unto Caesar the things that are Caesar's, and unto God the things that are God's" (Mark 12:17 ASV). The state, he says, is concerned with the external welfare of the citizen, his liberty, and prosperity. The church is concerned with his spiritual welfare. So long as the two do not conflict, each should tolerate the other.

Locke acknowledges that such toleration may give rise to problems of its own, and his libertarianism is not by any means absolute as he addresses these subsidiary issues using the same logic. He does not advocate the toleration of atheism, since he regards religion as the basis of the covenants that hold society together. He also says that the state should not tolerate sects that themselves fail to tolerate other sects. Nonetheless, the liberalism of the Letter was seen as so radical when it was first published in 1689 that it came under fierce attack from members of the Anglican clergy.