Nikon became Patriarch of Moscow in 1652 already convinced that centuries of hand copying and local variation had let Russian liturgical practice drift away from authentic Greek Orthodoxy, the church from which Rus had originally received Christianity in 988. With the strong support of Tsar Alexis, he ordered new service books printed to match contemporary Greek and Kyivan exemplars rather than the older Muscovite texts, and he pressed a set of ritual changes into universal use: the three-finger sign of the cross in place of the traditional two-finger gesture, the spelling Iisus rather than Isus for the name of Jesus, a changed number of prosphora loaves used in the liturgy, and a reversal in the direction certain processions moved around the church.
Resistance from clergy and laity who considered the older forms not merely customary but doctrinally bound up with salvation itself led Nikon and his successors to seek formal conciliar backing. Councils held in Moscow in 1666 and 1667, attended by visiting Greek and other Eastern patriarchs alongside the Russian bishops, approved the reformed texts and rites as the only legitimate standard and went further than simply correcting practice, pronouncing anathema against the old rite itself and against anyone who continued to use it, treating the older forms as a heresy rather than merely a local custom to be phased out gently.