Born to a family of petty Ruthenian nobles (szlachta), Protasewicz worked as a scribe, notary, and secretary at the chancellery of the Grand Duchy of Lithuania until his appointment of bishop.

[1] Protasewicz was born in a family of Ruthenian nobles (szlachta)[2] in a small village of Shushkova[3] (Belarusian: Шушкова, Polish: Szuszkow) near Kraysk in the Minsk Voivodeship of the Grand Duchy of Lithuania.

[1] He was a leading member of the Lithuanian delegations sent to the Polish Great Sejm to negotiate the Union of Lublin.

[5] He was criticized by his contemporaries, including Augustinus Rotundus and Stanislaus Hosius, for neglecting religious matters and allowing the Reformation to spread.

[5] Protasewicz soon began to combat the Protestantism by calling two diocesan synods, disciplining priests, and improving the Cathedral School of Vilnius.

[8] In 1573, Protasewicz reburied remains of Grand Duke Vytautas (died in 1430) by the altar of the Holy Cross in Vilnius Cathedral and built a tomb sponsored by Queen Bona Sforza.

[9] In 1574, after death of Samogitian bishop Jurgis Petkūnas, Archbishop of Gniezno Jakub Uchański attempted to promote his nephew.

[5] Already in 1553, the Jesuits offered to establish a college in Vilnius, but Grand Duke Sigismund II Augustus delayed due to the Livonian War, opposition of some Protestant nobles, and difficulties in finding the right personnel.

[12] With assistance from papal nuncio Giovanni Francesco Commendone and Bishop of Warmia Stanislaus Hosius, Protasewicz persuaded Sigismund to allow the school to be established.

[14] The Jesuits hoped that the new school would become their stronghold, preparing new generations of Catholic-educated activists for future religious work.

[20] The new school year in October 1570 began with 122–160 students based on the classical curriculum of trivium and quadrivium.