A fanatical group all together have a comfortable feeling that they’re all friends with one another. They are all very much excited about the same thing. You can see it in any political party. There’s always a fringe of fanatics in any political party, and they feel very cozy with one another; and when that is spread about and is combined with a propensity to hate some other group, you get fanaticism well developed.

Bertrand Russell
Bertrand Russell (1872-1970) English mathematician and philosopher
Interview by Woodrow Wyatt, BBC TV (1959)
    (Source)

Collected in Bertrand Russell's BBC Interviews (1959) [UK] and Bertrand Russell Speaks His Mind (1960) [US]. Reprinted (abridged) in The Humanist (1982-11/12), and in Russell Society News, #37 (1983-02).