Upon the request of Prince Cheitmar or Hotimir[2] of Carantania to Christianize his people, Bishop Vergilius dispatched Modestus around the year 755, together with four priests and a deacon "and other inferior clerks"[3] as a missionary with the rank of a chorepískopos (Ancient Greek: Χωρεπίσκοπος), i.e. a chorbishop responsible for the people in the countryside.
Cheitmar's predecessor Borut had accepted Bavarian overlordship about 740 and Modestus' missionary work in Carantania was meant to stabilise the country against the invading Avars.
It was described in the Conversio Bagoariorum et Carantanorum[6] written around 870 as a memorandum of the Salzburg archbishop Adalwin in a court hearing before the East Frankish king Louis the German against Bishop Methodius, the apostle of the Slavs in the Frankish Principality of Lower Pannonia and in Great Moravia.
In the document, the Archdiocese of Salzburg emphasised the achievements of Modestus as an argument of their merits in converting the Slavs.
Due to his success in converting the pagan Carantanian Slavs to Christianity, Modestus was honoured by the popular denomination "Apostle of Carinthia".