The letter, most probably written by Ambrose of Milan, was sent in the name of the Council of Aquileia in September of 381. It is addressed to all three emperors but most probably it was intended only for Theodosius who wielded power over the East. Strangely, the bishops for the Council of Aquileia seem to not know that the Antiochene schism was already settled at the council of Constantinople in July of 381. Ambrose and the Italian bishops learnt about that only later but were dissatisfied with the resolution, therefore they wrote another letter to the emperor (Epistula extra collectionem 9 = Maur. 13). For the discussion about this correspondence see Liebeschuetz 2010: 231-233.