Josaphat's superior, Samuel, never accepted unity with Rome, and looked for a way to fight against Roman Catholicism and the Uniats, the name given those who brought about and accepted the union of the Churches. Learning of Samiel's work, and fearing the physical and spiritual damage it could cause, Josaphat brought it to the attention of his superiors. The archbishop of Kiev removed Samuel from his post, replacing him with Josaphat.
Famous preacher. Worked to bring unity among the faithful, and bring strayed Christians back to the Church. Bishop of Vitebsk. Most religious, fearing interference with the natively developed liturgy and customs, did not want union with Rome. Bishop Josaphat believed unity to be in the best interests of the Church, and by teaching, clerical reform, and personal example Josaphat won the greater part of the Orthodox in Lithuania to the union. Never completely suitable to either side, Roman authorities sometimes raised objection to Josaphat's Orthodox actions. Archbishop of Polotsk, Lithuania in 1617.
While Josaphat attended the Diet of Warsaw in 1620, a dissident group, supported by Cossacks, set up an anti-Uniat bishops for each Uniat one, spread the accusation that Josaphat had "gone Latin," and that his followers would be forced to do the same, and placed a usurper on the archbishop's chair. Despite warnings, John went to Vitebsk, a hotbed of trouble, to try to correct the misunderstandings, and settle disturbances. The army remained loyal to the king, who remained loyal to the Union, and so the army tried to protect Josaphat and his clergy.
Late in 1623 an anti-Uniat priest named Elias shouted insults at Josaphat from his own courtyard, and tried to force his way into the residence. When he was removed, a mob assembled and forced his release. Mob mentality took over, and they invaded the residence. Josaphat tried to insure the safety of his servants before fleeing himself, but did not get out in time, and was martyred by the mob. His death was a shock to both sides of the dispute, brought some sanity and a cooling off period to both sides of the conflict."
-- Biography from SQPN
** If I'm not mistaken, our brethren using the 1962 Roman Missal observe St Josaphat's memorial on 14 November.
"Christ the Lord passed on to his apostles the task he had received from the Father: I have been given all authority in heaven and on earth. Go, therefore, and make disciples of all nations. He wanted the apostles as a body to be intimately bound together, first by the inner tie of the same faith and love which flows into our hearts through the Holy Spirit, and, second, by the external tie of authority exercised by one apostle over the others. For this he assigned the primacy to Peter, the source and visible basis of their unity for all time. So that the unity and agreement among them would endure, God wisely stamped them, one might say, with the mark of holiness and martyrdom.
Both these distinctions fell to Josaphat, archbishop of Pollock of the Slavonic rite of the Eastern Church. He is rightly looked upon as the great glory and strength of the Eastern Rite Slavs. Few have brought them greater honor or contributed more to their spiritual welfare than Josaphat, their pastor and apostle, especially when he gave his life as a martyr for the unity of the Church. He felt, in fact, that God had inspired him to restore worldwide unity to the Church and he realized that his greatest chance of success lay in preserving the Slavonic rite and Saint Basil's rule of monastic life within the one universal Church.
Concerned mainly with seeing his own people reunited to the See of Peter, he sought out every available argument which would foster and maintain Church unity. His best arguments were drawn from liturgical books, sanctioned by the Fathers of the Church, which were in common use among Eastern Christians, including the dissidents. Thus thoroughly prepared, he set out to restore the unity of the Church. A forceful man of fine sensibilities, he met with such success that his opponents dubbed him 'the thief of souls.'"
-- From The Liturgy of the Hours