Due to the partition of the Polish Commonwealth five Uniat dioceses: metropolitan, Polock, Wlodzimiersk, Luck and Chelm - were included into the Russian Empire. The Russian Orthodox Church was malevolent towards Uniats. It was a common belief that Uniats should return to the Orthodox Church. The first action of conversion of Uniats was conducted in Volhynia and Podole in the years 1793-1794. As a result of the missionary action, supervised by bishop Wiktor Sadkowski, about 200 Uniat parishes, with about 1 million people, were converted to the Orthodox Church faith. Tsar Paul II, although not very fond of Uniats, did not discriminate them. During his reign, Luck and Brzesk bishoprics were restituted and the Uniat Church had 1500 parishes, 80 Basilian monasteries and 1428000 faithful. Alexander I did not fight against Uniats but Nicholas I conducted a different policy. He believed that Uniat clergy of Byelorussian or Ukrainian nationality belonged to the eastern cultural sphere, so he spared no efforts to include them into the Orthodox Church. The Uniat Church was to be devoid of Latin influences. Restoration of full eastern ceremony in Uniat churches was to be a preliminary stage of the peaceful unification with the Orthodox Church. Józef Siemaszko was to be the main protagonist of this process. In 1827 he worked out a memorial on the state of the Greek-Catholic Church in Russia being a plan of the unification of the Uniat and Orthodox Churches. The project was supported by Nicholas I. The first significant movements aiming at the separation of Uniats from Catholics were undertaken in 1828. Changes introduced in the years 1828-1829 led to the separation of the Uniat and Catholic hierarchies, organization of the Uniat system of education and the strengthening of the pro-Orthodox mood among the clergy and the faithful of the Greek-Catholic Church. Works on the preparation of the unification synod were accelerated after the November Insurrection. Participation of Catholics and Uniats in the 1830-1831 insurrection gave Tsar Nocholas I a pretext for the introduction of some restrictions imposed on the Uniat faithful. In the years 1835-1839 the organizational structure was rebuilt, Orthodox liturgical books were introduced, Uniat churches' interiors were modeled after Orthodox churches' ones. From 1837 on all issues concerning the Uniat Church were to be handed over to the prosecutor of the Synod. On March 3, 1838 Józef Siemaszko started presiding over the Greek-Catholic College. On February 12, 1839 Uniat bishops, 24 members of higher clergy and some lay representatives gathered at a council. They proclaimed and signed an act of unification. A list of 1305 members of clergy declaring their support for the idea of the unification was enclosed. The unification with the Orthodox Church was not supported by 593 members of the Uniat clergy. On March 25, 1839 the Synod of the Russian Orthodox Church admitted Uniat clergy and lay people to its circle. The Greek-Catholic College for Clergy started to be called Lithuanian-Byelorussian College for Clergy and it came under the Synod's supervision. It was headed by Józef Siemaszko - promoted to the rank of Archbishop.
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