This paper explores the historical formation of the deontology of public officers, and of ambassadors in particular. After introducing the Latin concept of officium (office/duty), we will comment briefly on the gradual conceptualization and organization of the Christian Church around different ministers and figures, each with its specific status, duties and functions, as clearly attested by the Christian literature de officiis (from Ambrose to Isidore, to William Durand). We will then focus on two jurists, Lucas de Penna († c. 1390) and Bernard du Rosier (1400-1475) who played important roles in modelling the public figure of the ambassador and his professional, moral and juridical duties; Penna and Rosier were among the first authors to deal explicitly with this issue, thus paving the way for subsequent thought. Their texts, informed by their active participation in political life, built on the legacy of the abovementioned Christian tradition de officiis in defining the role of the ambassador, and made express references to the Bible, theological works and canon law. Analysis of these texts thus provides an important perspective for an appreciation of the contribution made by Christianity to the shaping of one of the leading actors on the stage of international law.