In 1635 he became bishop of
Mstislavl,
Orsha, and
Mahiliow. Following the
Union of Brest, the eparchy of Mstislawl, Orsha, and Mahiliow became the only Eastern Orthodox eparchy within the Grand Duchy of Lithuania. In 1647 he became the metropolitan of Kyiv. It happened just before the
Khmelnytsky Uprising, a time of uncertainty in Ukraine and in the Ruthenian church. Kossów himself was strongly opposed to union with
Tsardom of Russia, and also against unreserved alliance with
Poland. He condemned the 1654
treaty of Pereyaslav between the Cossack Hetmanate and the Tsardom of Muscovy. Despite being critical of some of the policies of the
Ukrainian Cossacks, he attempted to rectify the situation and along with the
Patriarch of Jerusalem Paiseus gave
Bohdan Khmelnytsky a hero's welcome when he entered Kyiv on 2 January 1649 (Old Style 23 December 1648). Kossów strived for an independent Ruthenian Orthodox Church that would be only under the jurisdiction of the
Patriarch of Constantinople, and he fought against the subjugation of the Kyiv metropolate to Moscow. In 1650 Sylvestr headed a delegation of the Orthodox clergy and Kyiv voivode
Adam Kysil to the Sejm in
Warsaw and contribute to ratification of the 1649
treaty of Zboriv. ==Bibliography==