Avvakum (1621-1682) was a Russian archpriest who lead the opposition ot Patriarch Nikon's reforms of the Russian Orthodox Church.
Starting in 1652 Nikon, as Patriarch of the Russian Church, initiated a wide range of reforms in Russian liturgy and theology. These reforms were msotly intened to bring the Russian Church into line with the other Orthodox Churches of Eastern Europe. Avvakum and others rejected strongly these changes. They saw them as a corruption of the Russian Church, which was the true Church of God. The other Churhces were more closely related to Constantinople in their litrugies and Avvakum argued that Constantinople fell to the Turks because of these heretical beliefs.
For his opposition to the refroms Avvakum was bruned at the stake by the Russian government. Groups rejecting the changes continued, however, and they became refred to as the Old Believers. Avvakum's famous autobiograhpy survived him and it is an excellent description of the schism in the Church.