Rensi taught as a professor at the University of Genoa and was considered a proponent of Relativism and a supporter of the Conservative Revolution in Italy. In his 1920 tract Filosofia dell'autorità, he argued that, because different worldviews cannot be reconciled intellectually, there needs to be a single political authority backed by physical force in order to establish order in society. On this ground he was initially a backer of the then nascent Fascist movement. By 1925, however, with his work Apologia dell'ateismo, he opposed Benito Mussolini and was counted among the supporters of Benedetto Croce, who wrote a manifesto against Fascism the same year. In 1927 he was given garden leave from his lectureship and arrested for a time in 1930. Finally he was dismissed from his post in 1934, after having published further critical writing. In reference to his opposition, written upon his tombstone is "Etiamsi omnes, ego non".