From the 1910s onwards, the British secret state – notably MI5 and the Metropolitan Police Special Branch – put many atheist scientists, writers, and journalists under surveillance. This talk explores the dangers perceived to rest amongst non-religious people – ranging through anti-imperialism, the bomb, and the threat of sex – and the ways in which the humanist movement tried to claim respectability. By 1990, the problem had shrunk as the Cold War ended, sexual freedoms grew, and being without religion was popularised and normalised. Being humanist is now less revolutionary than once it was perceived. But does it retain a radical edge?