In the nineteenth and early twentieth centuries, widespread Catholic commentary cast a congeries of prophets, millenarians, freethinkers, metaphysicians, and other (ir)religious outsiders as an indictment of Protestantism in America. To Catholics, Mormons and Millerites, atheists and agnostics, Spiritualists and Christian Scientists were the exegetical and educational products of Protestantism. And mainstream Protestant reactions to these groups exposed the contradictions of Protestant power and anti-Catholic discourse in America. Catholics argued that proliferating religious radicals ultimately belied Protestants’ portrayals of their own exegetical, intellectual, and politico-religious freedom from Catholic oppression. Recovering Catholic commentary on religious outsiders and Protestantism in America helps correct the historiographical neglect of Catholic responses to anti-Catholicism, present oft-obscured historical Catholic perspectives on American religious history, recover a polemical dialogue where historians have offered a Protestant monologue, and qualify the historical cogency of anti-Catholic discourse in America. Most importantly, this study reveals a rare instance in which one marginalized religious group used other marginalized religious groups to interrogate and critique, rather than appeal to and deflect criticism from, a religious mainstream.