The diplomatic activity of Jacques Bongars, Jean Hotman, and Étienne de Sainte-Catherine – all French agents to the imperial Protestant princes between 1589 and 1620 – may be overlooked in French historiography, but in fact these three men were at the very centre of an intense epistolary activity whose content and chronology appear to transcend traditional diplomatic relationships. Their papers, which have been preserved exceptionally well, testify to the depth of their involvement in fulfilling their diplomatic duties, but above all to a prolific personal correspondence surpassing the requirements of their diplomatic charges. The study of their passive correspondence reveals a wide and interconnected network, embracing both Rhenish Protestant society and, on a wider scale, the Republic of Letters. The extent of their exchanges, transcending political and historical erudition, as well as their permanence, and the political charges of several of their correspondents, are suggestive of a porosity between diplomatic practice, confessional preoccupation, and politico-erudite networks, and thus of the influence exercised by Bongars, Hotman, and Sainte-Catherine on political and diplomatic decision-making both in France and in the Empire.