Old Believers are Russian Orthodox Christians who rejected the liturgical reforms Patriarch Nikon of Moscow introduced beginning in 1652 to align Russian rites with contemporary Greek Orthodox practice, a split Russians call the Raskol, literally a cleaving apart. Nikon and Tsar Alexis both believed Russian liturgical practice had drifted from its Greek origins through centuries of scribal error and local variation, and Nikon's reforms aimed to correct that drift by standardizing Russian texts and rituals against current Greek models, a project both men regarded as restoring rather than innovating.
Opponents read the same reforms as an assault on the faith itself rather than a technical correction, since the specific practices at stake, the number of fingers used to make the sign of the cross, the spelling of Jesus's name, the direction of liturgical processions, were understood by traditionalists as inseparable from correct worship rather than negotiable custom that could be adjusted without consequence. Opposition to Nikon's reforms was led by a group of Muscovite priests, most famously the archpriest Avvakum Petrovich, whose own account of his sufferings and eventual execution became a founding text of Old Believer identity and Russian religious literature more broadly.
Church councils culminating in 1666 and 1667 officially endorsed Nikon's liturgical reforms and formally anathematized the dissenters, and several leading Old Believers, including Avvakum himself, were subsequently executed by the state, a severity that both confirmed the schism's permanence and gave the emerging Old Believer movement its first generation of martyrs.