The US Catholic Church was for most of its history—and, in many places, still is—a working-class church. The choice for worship by successive waves of immigrants, from the Irish to the Polish to the Mexican, the Church, once it had created an institutional presence, welcomed “these strangers in a strange land.” These immigrants play a major role in creating and sustaining parishes that served both as a soul-sustaining refuge and, in many cases, a way station to the outside world. James Cardinal Gibbons, having learned from the central role that Irish workers played in the Knights of Labor and protests against the excommunication of the radical New York priest, Edward McGlynn, persuaded the Vatican to take a relatively liberal stance toward the “social question” in the United States. Rerum Novarum, the 1891 papal encyclical, condemned socialism and competitive capitalism, but more significantly asserted the “natural” right of workers to form unions as well as to have a living wage. It was within this religious legitimation of unionism that Irish Catholics came to prominence in the American Federation of Labor, that Monsignor John A. Ryan created a US Catholic social justice intellectual tradition, and that US bishops adopted the 1919 Program for Social Reconstruction. The Catholic labor moment came when the Church, led by the National Catholic Welfare Conference’s Social Action Department, midwestern bishops, and labor priests, not only supported the Congress of Industrial Organizations (CIO), but consistently pushed the New Deal to implement the 1919 program. Philip Murray, the CIO’s Catholic president, led the expulsion of the Communist-led unions when the Communist Party, in the Wallace campaign, threatened both the country and everything the CIO had built. On the one hand, this Catholic labor moment dissolved in an overdetermined mixture of complacency, capitalist growth, and anti-Communism. On the other, a direct line can be traced from California’s labor priests to the Spanish Mission Band to Cesar Chavez and the formation of the United Farm Workers. It took time for the official Church to support the farm workers, but once that happened, it was all in: the support the Church, at all levels, gave them far exceeded anything it had done previously to implement Rerum Novarum.