John Kuntsevych, who was born in 1580 in the Volhynian region of western Ukraine, did not become "Josaphat" until he became a monk. Additionally, because he was born to Orthodox Christian parents whose church had lost communion with the Pope, he was not initially a complete member of the Catholic Church.

Unity had been maintained for a while following the Ecumenical Council of Florence in the 15th century, even though the Eastern churches started to break away from the Holy See in 1054. However, the union was already breaking down before the Turkish conquest of Byzantium in 1453 due to social, political, and religious disagreements. Many Slavic Orthodox Christians had developed a strong anti-Catholic stance by the time of John.

Latin missionaries made an effort to reunite with the several Eastern patriarchs during this period. The strategy was dangerous because it occasionally politicized the faith and widened existing rifts. However, it did result in some noteworthy achievements, like as the Union of Brest in 1596, which brought John's own Ruthenian Church back together.

John had the option of getting married after receiving training as an apprentice for a merchant. However, the strictures and profound spirituality of traditional Byzantine monasticism drew him in. In 1604, he entered a Ukrainian monastery under the monk’s name of Josaphat.

Amid the Catholic Church's "Counter-reformation," the young monk was embarking on a bold mission: to reintegrate the Eastern Orthodox heritage with its authority. Soon, he would live and die for the unity of the churches as a priest, then an archbishop, and finally a martyr.

Josaphat opposed any attempt to undermine the Eastern Catholic churches' customs while simultaneously rejecting the anti-Western attitudes of many of his fellow citizens. He created apologetics and catechisms in response to the people's pressing pastoral needs, addressed the impoverished, and enacted long-overdue changes in the clergy.

Many Orthodox Christians trusted the archbishop because of his example of life and passion for soul care, which demonstrated the importance of the church's unity. However, his purpose was fundamentally contentious, and others were misled by vile rumours and threats launched against him. Opponents planned the consecration of a rival archbishop in 1620.

Josaphat bemoaned the start of the attacks that would ultimately result in his death as hostilities between supporters and opponents grew more intense. An Orthodox priest attempted to enter the archbishop's home in 1623 while yelling obscenities outside. The man organized a crowd in the town after Josaphat had him removed. When they arrived, they threatened the archbishop's friends and staff and demanded his life. Unable to flee, Josaphat died praying for the people who shot, decapitated and dumped his body in a river.

Five years later, St. Josaphat's body was found to be incorrupt. It is noteworthy that the Orthodox Archbishop Meletius, the saint's former adversary, ultimately made peace with the Catholic Church. In 1867, St. Josaphat was declared a saint.

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