The wonder is that Locke seemed blissfully unaware, or unconcerned, that Islam was not benign and was therefore incompatible with political liberalism because it was a political religion which spread by the sword, not by the dictates of conscience.
A manuscript titled “Reasons for Tolerating Papists Equally with Others,” written in Locke’s hand in 1667 or 1668, has just been published for the first time, in The Historical Journal of Cambridge University Press. The document challenges the conventional view that Locke shared the anti-Catholicism of his fellow Protestants. Instead, it offers a glimpse into the radical quality of his political liberalism, which so influenced the First Amendment and the American Founding. “If all subjects should be equally countenanced, & imployed by the Prince,” he wrote, “the Papist[s] have an equall title.” ...
In his first major treatise supporting religious liberty, An Essay Concerning Toleration (1667), Locke constructs an argument, a defense of the rights of conscience, that he will build upon for the rest of his life. He argues that magistrates have no right interfering in religious beliefs that pose no obvious threat to the social order: “In speculations & religious worship every man hath a perfect uncontrolled liberty, which he may freely use without or contrary to the magistrate’s command.” The challenge of accommodating different religious traditions, including Roman Catholicism, is front and center. “If I observe the Friday with the Mahumetan, or the Saturday with the Jew, or the Sunday with the Christian, . . . whether I worship God in the various & pompous ceremonies of the papists, or in the plainer way of the Calvinists,” he wrote, “I see no thing in any of these, if they be done sincerely & out of conscience, that can of itself make me, either the worse subject to my prince, or worse neighbor to my fellow subject.” ...
What Locke found intolerable was not Catholic theology per se but rather the agents of political subversion operating under the guise of religious obedience. As he put it in the newly discovered manuscript: “It is not the difference of their opinion in religion, or of their ceremonys in worship; but their dangerous & factious tenets in reference to the state . . . that exclude them from the benefit of toleration.” On this point, Locke could be as tough on Protestants as he was on Catholics. ...
Political philosopher Greg Forster insightfully observes that Locke “towers over the history of liberalism precisely because virtually everything he wrote was directed at coping with the problem that gave birth to liberalism — religious violence and moral discord.” ...
America’s experiment in human liberty and equality is profoundly Lockean. It is also, in some important respects, deeply Christian. Locke believed that the gospel message of divine mercy — intended for all — implied political liberalism. The founder of Christianity, he wrote, “opened the kingdom of heaven to all equally, who believed in him, without any the least distinction of nation, blood, profession, or religion.”
It would be hard to conceive of a better doctrine on which to build a more just and humane society. A revival of Lockean liberalism would do much to tame the hatreds now afflicting the soul of the West.