With the central theme of the book being the nature of a true revolutionary, the reflections on religion and rebellion proved to be ideologically suitable and successful. "The Gadfly" was exceptionally popular in the Soviet Union, the People's Republic of China, and Iran, exerting a large cultural influence. In the Soviet Union, "The Gadfly" was compulsory reading and the top best seller; indeed, by the time of Voynich's death, "The Gadfly" is estimated to have sold 2,500,000 copies in the Soviet Union alone. Voynich was unaware of the novel's popularity, and did not receive royalties, until visited by a diplomat in 1955. In China, several publishers translated the book, and one of them (China Youth Press) sold more than 2,050,000 copies. It was banned, however, after the Sino-Soviet split. Irish writer Peadar O'Donnell recalls the novel's popularity among Republican prisoners in Mountjoy Prison during the Irish Civil War.