The active phase is the middle phase of a mass movement. It is led by fanatics and dominated by true believers. For this reason, Eric Hoffer regards it as the ugliest phase. The True Believer focuses almost exclusively on a mass movement’s active phase.

As a doctrine, Communism follows the philosophy of 19th-century author Karl Marx, who criticized capitalist societies and called for a worldwide revolution of the industrial working class. Hoffer, however, downplays the importance of ideologies. In The True Believer, Communism refers to the specific mass movement that seized power in Russia in 1917, which ushered in the Union of Soviet Socialist Republics (U.S.S.R.) and, by 1951, appeared as the chief rival of the Western democracies.

Fanaticism describes the worldview of the true believer, particularly during a mass movement’s active phase. Some people might dabble on the fringes of a movement, but the fanatic surrenders his or her entire being to it. The conversion from autonomous individual to true believer is almost always a passionate act of self-renunciation. This produces in the true believer a wholesale submission to the movement, an unquestioning devotion to the cause, and a willingness to commit any action, including self-sacrifice, should the movement require it.