This dissertation traces the evolution of black abolitionism in colonial North America and the United States from 1740 to 1841. Focusing primarily on reformers, theologians, and activists, it examines specifically the ways in which spiritual beliefs shaped black opposition to slavery. It places black abolitionists in an international context and analyzes the transatlantic connections they developed and maintained in their battle against slavery and prejudice. Inspired by eighteenth-century pietistic revival, a West African cosmological heritage, and the Enlightenment emphasis on natural rights, men and women of African descent began protesting slavery publicly during the colonial era. With the onset of the American Revolution, they located republican egalitarianism within a sacred framework and underscored the contradiction inherent in a slaveholding polity allegedly predicated on Protestant Christianity. After 1800, many black activists adapted the pietistic model of itinerancy and evangelism to agitate against both slavery and racial discrimination. During the 1820s, black antislavery reformers, disillusioned by the nation's rejection of abolition and angered by the American Colonization Society's 1817 plan to send free blacks to Africa, embraced more radical measures. By 1829, they demanded immediate emancipation and after 1830 consolidated their strategies into a full-fledged radical movement. This study relies on three investigative methods. First, it employs untapped or underutilized archival and textual sources that uncover a biracial transatlantic network of activism as early as the 1780s. Second, it contextualizes familiar documents including newspaper reports, fugitive slave advertisements, conversion narratives, and public orations more finely. Finally, it draws on black intellectual productions rarely used in discussions of abolitionism. This dissertation intervenes in the historiography of African American history and culture, abolitionist literature, and transatlantic intellectual history. By placing people of African descent central to the emergence of abolitionism, it offers an important interpretation for how one of the most significant social reform movements in American history developed. The dissertation argues that the call for immediate emancipation that gained currency after 1830 originated in black reformers' collective efforts to put into practice their spiritual convictions.