Although the civil rights movement has long been framed as a pivotal turning point in twentieth-century U.S. religious history, comparatively little attention has been directed to the role of religion in what has been termed “the long segregation movement.” Likewise, Catholic historians tend to emphasize the exceptional few priests, sisters, and lay people committed to interracial justice over and against the majority of white Catholics who either opposed integration or objected to the means by which it would be achieved. This article argues that, in order to fully understand U.S. Catholicism in the twentieth century, scholars must reckon with the ways racial whiteness shaped the Catholicness of white Catholics. It takes as its primary source more than six hundred letters written by white Catholics outraged and disgusted over the Archdiocese of Chicago's apparent support for desegregation between 1965 and 1968. These letters not only illuminate the inseparability of religion and race, but they also reveal that white Catholicism itself operated as a religio-racial formation in the lives of white Catholics. Given the overwhelming white Catholic (and white religious) resistance to integration, this article argues that the long segregation movement and massive resistance to desegregation ought to be included as signal events in the telling of U.S. Catholic and U.S. religious history.