"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.