A Letter Concerning Toleration by John Locke was originally published in 1689. Its initial publication was in Latin, though it was immediately translated into other languages. Locke's work appeared amidst a fear that Catholicism might be taking over England, and responds to the problem of religion and government by proposing religious toleration as the answer. This "letter" is addressed to an anonymous "Honored Sir": this was actually Locke's close friend Philipp van Limborch, who published it without Locke's knowledge.
One of the founders of Empiricism, Locke develops a philosophy that is contrary to the one expressed by Thomas Hobbes in Leviathan, in supporting toleration for various Christian denominations. Hobbes did allow for individuals to maintain their own religious beliefs as long as they outwardly expressed those of the state, however, and it has been argued that Locke's rejection of Catholic Imperialism was the ultimate basis for his rejection of government's interest in spiritual salvation.
Unlike Hobbes, who saw uniformity of religion as the key to a well-functioning civil society, Locke argues that more religious groups actually prevent civil unrest. Locke argues that civil unrest results from confrontations caused by any magistrate's attempt to prevent different religions from being practiced, rather than tolerating their proliferation. Locke's primary goal is to "distinguish exactly the business of civil government from that of religion." He seeks to persuade the reader that government is instituted to promote external interests, relating to life, liberty, and the general welfare, while the church exists to promote internal interests, i.e., salvation. The two serve separate functions, and so, must be considered to be separate institutions.
For Locke, the only way a Church can gain genuine converts is through persuasion and not through violence. This relates to his central conclusion, namely, that the government should not involve itself in care of souls. In support of this argument he presents three main reasons: (1) individuals, according to Locke, cannot divest control over their souls to secular forces, as God does not appoint the magistrate; (2) force cannot create the change necessary for salvation, because while it can coerce obedience, it cannot change one's beliefs; and (3) even if coercion could persuade someone of a notion, it would not help with ensuring salvation, because there is no reason to believe that magistrates are reliable judges of religious truth.
Locke argued that atheists should not be tolerated because 'Promises, covenants, and oaths, which are the bonds of human society, can have no hold upon an atheist'. There is, however, a passage added in a later edition of the Essay concerning Human Understanding, where Locke perhaps questions "whether 'atheism' was necessarily inimical to political obedience."
The Roman Catholic Church cannot be tolerated either, according to Locke, because 'all those who enter into it do thereby ipso facto deliver themselves up to the protection and service of another prince'. If this Church were tolerated, a magistrate would have to abide by the settling of a 'foreign jurisdiction' in his own country and see its followers 'listed, as it were, for soldiers against his own Government'. However, at least one scholar has argued that "The combination of Locke’s comments in the Letter suggest that during [its] composition ... Locke was once again struggling over how to discriminate between the series of associated political principles which for him made Catholics intolerable, and the religious worship and other religious beliefs of Catholics which deserved toleration."
Toleration is central to Locke's political philosophy. Consequently, only churches that teach toleration are to be allowed in his society. Locke’s view on the difficulty of knowing the one true religion may suggest that religion is not personally important to Locke, but it also may point to the deep uncertainties surrounding religious belief in a time of political and intellectual conflict. In contrast, Locke’s view on atheism suggests that he was far from considering religion as unimportant. As an empiricist, he took practical considerations into account, such as how the peace of civil society will be affected by religious toleration. A close reading of the text also reveals that Locke relies on Biblical analysis at several key points in his argument.
There were immediate responses from the High Church Anglican clergy, published by Thomas Long and Jonas Proast. Long believed the letter was written by an atheistically disguised Jesuit plot for the Roman Catholic Church to gain dominance by bringing chaos and ruin to church and state. Proast attacked the Letter and defended the view that the government has the right to use force to cause dissenters to reflect on the merits of Anglicanism, the True Religion. Locke's reply to Proast developed into an extended, controversial exchange.