The Russian Orthodox Church is the largest autocephalous Orthodox Church in the world. Officially, the Church traces its history to the introduction of Christianity in Rus' by Volodymyr the Great. However, in reality the existence of the Church dates from the sacking of Kyiv by the Mongols in 1240; at that time the Kyivan metropoly was transferred from Kyiv to Vladimir on the Kliazma. During the expansion of the Russian Empire from the 16th century onward, the Church served as an agent of Russification; this was particularly true in Ukraine, where the Kyiv metropoly was banned from cultivating its traditional liturgical style and language, and forced to conform to Russian rituals. During Soviet times, the Communist Party of the Soviet Union controlled the Church closely. There was a Council on Religious Affairs at the Council of Ministers of the USSR and similar councils in all the Soviet republics, through which the activities of the Church were controlled. When other churches were banned, as, for example, the Ukrainian Autocephalous Orthodox Church in the 1930s and the Ukrainian Greek Catholic Church in 1946, the Russian Orthodox Church absorbed their priests, property and parishes. Starting in Stalinist times, then, the Russian Orthodox Church became another tool of control used by the Soviet regime. After the collapse of the USSR and the re-establishment of the Greek Catholic and Ukrainian Autocephalous Orthodox Churches, the Russian Orthodox Church lost many parishioners. All the same it remains the largest single church in Ukraine today.