Epstein was born in Ruzhany, Grodno Guberniya, Imperial Russia (now in Belarus). His family was unsure of his exact birth date, but assigned him March 15, 1889, the second day of Purim. During his teenage years he became active in the Jewish territorialist movement. Beginning shortly before the 1905 revolution he was a cadre in the Zionist Socialist Workers Party in Białystok, Łódź, Warsaw, Kiev and Odessa. His activities included writing for party journals, organizing unions and Jewish cultural societies and, in Białystok manning an armed guard of the Jewish neighborhood during a pogrom. He served three intermittent periods in the Czarist prisons. Becoming dissatisfied with the ultimate aims of the territorialists he emigrated to America in 1913. He arrived at Ellis island on December 24, 1913.