Liberal religion, as a discernible movement, was far more a product of the nineteenth century than of the seventeenth and eighteenth centuries. “Liberalism,” like “secularism,” was a coinage of the nineteenth century, and the first group, the Unitarians, to self-describe as liberal Christians only took denominational shape in the 1820s and 1830s. Even in the case of the Unitarians, the label of “liberal Christian” was first pinned on them as a tag to mark their departure from the soundness of Calvinist orthodoxy, not as a proud party badge. Only in 1815 did William Ellery Channing formally embrace the appellation as an indicator of the Christian charity, broad-mindedness, and creedal flexibility – the disposition toward liberality – that he thought typified the nascent Unitarian movement. This essay, with British America its primary purview, necessarily details the background, not the foreground, of religious liberalism. It explores the intellectual and political frameworks out of which full-blown liberal religious movements and organizations – from Unitarianism to Reform Judaism to the Ethical Culture Society – subsequently emerged. It highlights the theological, political, cosmopolitan, and colonial roots of religious liberalism: the advent of deism and universalism, the development of church-state separation, and the process of liberal self-definition through conflict with religious opponents and through growing knowledge of the world's manifold religions.