In 1604, in his early 20s, Kuntsevych entered the Monastery of the Trinity (Church and monastery of Holy Trinity) of the Order of Saint Basil the Great in Vilnius, at which time he was given the religious name of Josaphat. Stories of his sanctity rapidly spread and distinguished people began to visit the young monk. After a notable life as a layman, Rutsky also joined the Order. When Josaphat was ordained to the diaconate, his regular services and labor for the Church had already begun. As a result of his efforts, the number of novices to the Order steadily increased, and under Rutsky – who had meanwhile been ordained a priest – a revival of Eastern Catholic monastic life began among the Ruthenians (Belarusians and Ukrainians). In 1609, after private study under Jesuit Valentin Groza Fabricy, Josaphat was ordained a priest by a Catholic bishop. He subsequently became the hegumen (prior) of several monasteries. On November 12, 1617, he was consecrated as the coadjutor archeparch for the Archeparchy of Polotsk. He succeeded as archeparch in March 1618. During his episcopacy, the Saint Sophia Cathedral in Polotsk was rebuilt in 1618–1620.