His job allowed him time to explore the archives and libraries of Lisbon and other cities, where he collected and studied enough material to form the basis of a doctoral dissertation on the history of the Crusades in medieval Portugal.
In the meantime, Erdmann was recruited by the Monumenta Germaniae Historica (MGH), the prestigious historical institute dedicated to the critical editing and publishing of sources relating to German history in the medieval period.
As an academic researcher (wissenschaftlicher Mitarbeiter) at the MGH, Erdmann was assigned the task of preparing editions for a number of important manuscript letter collections from the eleventh century relating to the history of the Investiture Controversy and Gregorian Reform.
During this period, Erdmann made a number of critically important contributions to the study of epistolary literature in the Middle Ages and the role letters exchanged among bishops, abbots, kings and other elites played in communicating political ideas.
Under normal circumstances, a position at the MGH would have led to a prestigious university chair in medieval history within a few years and no-one doubted that Erdmann was the brightest rising young star in his field.
Although Erdmann continued to work at the MGH, his liberal views and refusal to join or endorse the Nazi Party effectively blacklisted him from further academic advancement, particularly at the university level.