Taking as its starting point the visit of the American George Barrell Cheever to Europe in 1844, this chapter sets up a broad comparative framework for the analysis of nineteenth-century anti-Catholicism. Comparative approaches to the subject are as yet undeveloped in the face of the understandable tendency of scholars to root their work in the specificities of particular nations, regions and localities. There are however striking similarities in the chronologies of anti-Catholic movements in the North Atlantic world that highlight the value of studying them in parallel. Causation was complex and diverse, but the factor highlighted here is the context of religious competition between Roman Catholicism and expansionist Protestant evangelicalism. Consequences were paradoxical: constitutional links between church and state were weakened as national religious cohesion was promoted, and religious antagonisms stimulated the long-term advance of liberty and pluralism.