[1] In the twentieth century, Diefenbach became best known through his popular novel Die Wahrheit macht frei, a subsection of which, Arbeit macht frei, was later adopted as a slogan by various German and Austrian institutions, and most notoriously by the Nazis.

The novel was published in Bremen in 1873 (following a preprint in a Viennese newspaper the previous year).

The main hero is a gambler and fraudster who, through regular employment, succeeds in regaining the path of virtue.

Diefenbach’s most enduring legacy lies in his numerous academic writings on philology and comparative linguistics, notably his studies in Latin, German dialects and Celtic languages.

Much of the content of his Glossarium Latino-Germanicum was incorporated into revised editions of the Glossarium ad scriptores mediae et infimae latinitatis of Charles du Fresne, sieur du Cange.