The question is engaged of why political secularisms whose principles were articulated by the eighteenth century, only became operative in the twentieth century. The resistance of conservative religious actors is identified as a major factor. From there we round out our survey of ten secular principles by studying John Locke’s 1689 A Letter Concerning Toleration. Locke’s thought introduced the principles of toleration, belief-acts, disestablishment/neutrality, and reason. We focus on an obscure, but revealing, passage from Locke’s work we label the “override clause.”