He studied Danish history, and it was by critical articles on contemporary writers in this field that he attracted attention.
He obtained his education at the University of Copenhagen and was a favored student of historian Hans Gram. His most important work was a collection of Danish historical documents from the Middle Ages, titled Scriptores Rerum Danicarum Medii Œvi (1772–1774). He was unable to finish its publication, and the work was completed by Peter Frederik Suhm and others from original papers (1778).
He also collected material for a Danish dictionary.
A collection of his letters was published in 1794.
In 1745 he founded a society for the study of the language and history of Denmark, the Det kongelige danske Selskab for Fædrelandets Historie og Sprog (Royal Danish Society for Fatherland History and Language), and was its perpetual archivist.
Royal Swedish Academy of Sciences.