Josaphat joined the first monastery of the order of St Basil to be united to the Catholic Church: he was the first person to do so. He was ordained priest and, eventually and reluctantly, appointed bishop of Polotsk in 1617. Although Sigismund III Vasa, King of Poland and Grand Duke of Lithuania, supported the union, the local aristocracy were against it because it threatened their control of ecclesiastical benefices. Plotting with the Orthodox Patriarch of Jerusalem, who visited the region in 1621, they stirred up trouble and as a result Josaphat was murdered by a mob in 1623 while on a pastoral visit to Vitebsk.