The article reconstructs the circumstances and events of the well-known episode of 390, starting from the revolt in Thessaloniki. Ambrose could not have acted as a spokesman for the public indignation caused by the savage reprisal in Thessaloniki (of which only very few people could know). It is highly probable that he was the first to inform the Church council about the massacre. The public opinion for which he spoke in his letter was probably formed by himself. A new analysis of the letter (extra coll. 11) has revealed the true meaning of an underestimated passage consisting mostly of Biblical citations. The choice of citations, their order and context make it possible to consider this part of the letter as the central one. It is there that Ambrose formulated his demands: (1) Theodosius must acknowledge his sin and repent, (2) his repentance must be public, (3) he must repent of having murdered innocent people. Even a threat could be read between the lines: if Theodosius did not repent, God’s wrath would befall his descendants. Many opinions were uttered concerning the form and duration of Theodosius’ penitence. Comparing the situation with that described in Actus Silvestri, the author shows that even a short period of seven days could have been considered sufficient for the emperor who had committed a grave sin. Five years later Ambrose depicted Theodosius’ repentance in De obitu Theodosii in quite a different way. To support the succession of Theodosius’ son Honorius (who presumably inherited his fathers virtues), Ambrose represented the Emperor’s repentance as an act of humility ennobling him. Since then, the scenes of monarch’s repentance have developed into an important element of representing the power of an ideal Christian ruler.