Catholic social activism in the United States is the practical application of the notions of Catholic social teaching into American public life. Its roots can be traced to the 19th century encyclical Rerum novarum of Pope Leo XIII. Labor union movement The Catholic Church exercised a prominent role in shaping America's labor movement. From the onset of significant immigration in the 1840s, the Church in the United States was predominantly urban, with both its leaders and congregants usually of the laboring classes. Over the course of the second half of the nineteenth century, nativism, anti-Catholicism, and anti-unionism coalesced in Republican politics, and Catholics gravitated toward unions and the Democratic Party. The Knights of Labor was the earliest labor organization in the United States, and in the 1880s, it was the largest labor union in the United States. and it is estimated that at least half its membership was Catholic (including Terence Powderly, its president from 1881 onward). Effects of Rerum novarum This was the context in which Pope Leo XIII wrote an encyclical letter that articulated the teaching of the Church with a view to the “new things” of the modern world. In Rerum novarum (1891), Leo criticized the concentration of wealth and power, spoke out against the abuses that workers faced and demanded that workers should be granted certain rights and safety regulations. He upheld the right of voluntary association, specifically commending labor unions. At the same time, he reiterated the Church’s defense of private property, condemned socialism, and emphasized the need for Catholics to form and join unions that were not compromised by secular and revolutionary ideologies. Rerum novarum provided new impetus for Catholics to become active in the labor movement, even if its exhortation to form specifically Catholic labor unions was widely interpreted as irrelevant to the pluralist context of the United States. While atheism underpinned many European unions and stimulated Catholic unionists to form separate labor federations, the religious neutrality of unions in the U.S. provided no such impetus. American Catholics seldom dominated unions, but they exerted influence across organized labor. Catholic union members and leaders played important roles in steering American unions away from socialism. Fr. Edward McGlynn's work first in St. Stephen's Parish in New York and his later alliance and support of and activity with Henry George and the land value taxation for social justice movement. The onset of industrial organizing in the