The chronological and theological contours of conversion to Catholicism in the nineteenth-century United States evidence three waves. Beginning with John Thayer’s 1783 conversion from Congregationalism and continuing through the 1830s, conversions were scattered, and often from Reformed Protestantism. The 1840s through 1860s, the critical period for Catholic conversion, included converts from American Episcopalianism, riven by the Oxford movement, and from Transcendentalist and liberal Christian reformers dissatisfied with reform’s theological underpinnings. These converts became the agents of a new movement to convert Protestants. From 1870 through the early twentieth century, missionary priests, especially members of the Missionary Society of St. Paul the Apostle (Paulists), won thousands of converts, making conversion to Catholicism a viable choice for many more Americans.