Despite a century of debate and criticism, Marxism as a mass ideological practice has remained an elusive topic. This book examines Marxist socialism as a mode of understanding and self-understanding treasured and transmitted by thousands of anonymous militants. It focuses on the Parti Ouvrier Francais, the Guesdists, an archetypal movement of Marxism’s Golden Age before World War I, the period when Marxist socialism evolved from sect to mass movement. Thousands of French socialists adopted Marxism due to the effectiveness of vulgar Guesdist polemic rather than Marx’s profound theoretical works, and entire communities were converted to an austere but messianic socialism that still affects French politics today. This book traces the doctrine’s birth through conflict with liberals, proto-fascists, and anarchists; its making of a working class, and its attempted seduction of the middle class; and its confusion before the alternative social visions of the Catholic devout, racist nationalists, and feminists.