The main theme of Demons is how a respectable, well-meaning society, without realizing it, opens the way for a revolution made by people without scruples. Dostoevsky does not ask whether the conspirators will win; he asks what the town hands over to them.
The book answers in two ways. The first is filiation: the old liberal Stepan Trofimovich, who spent his life in poses, is the father of the conspirator and the teacher of the man everyone competes for. The second is mechanism: the governor yields to the logic of his own liberalism, the salon absolves Stavrogin with a single phrase, and the audience applauds the public shame of the homeland. Karmazinov even formulates the thesis: the Russian revolutionary idea consists in the negation of honor, and it is through the open "right to dishonor" that one can most quickly drag the Russian along.
Other themes turn around this one. Faith: Kirillov and the man-god, Shatov and the people who carry God, Tikhon and the complete atheist whom he prefers to the indifferent one. The family: Varvara who invents Stepan, Stepan who fathers Pyotr, the sons who do not obey the invention. Guilt: Stavrogin who knew and did not stop it, the "someone's" hand that knocks Liza down. And truth and lie: Pyotr who calibrates the size of the conspiracy for each listener, and Stepan who believes in himself when he lies.
For this reason the book is often read as a political novel, but its harshest charge is not against the doctrine; it is against those who use it without believing in it.