Letters: Engels To Marx 30 September 1847 https://marxistleninists.org/M&E/1847/letters/47_09_28.htm
Lenin: 178. DIRECTIVES TO THE ’ALGEMBA’ COMMISSION https://www.marxists.org/archive/lenin/works/1921/may/28c.htm
Comintern Archive: the 5 classics of Marxism-Leninism on religion http://ciml.250x.com/archive/5classics/english/5classics_on_religion.html