Short answers, pulled from the story.
The Old Believers movement began in 1653 when Patriarch Nikon of Moscow ordered the rewriting of Russian prayer books to align with Greek Orthodox practices. This directive triggered a chain reaction that fractured the Russian church and birthed the movement known as Old Believers.
Old Believers cross themselves with two fingers while the official church uses three fingers, recite the Alleluia twice instead of thrice, and use seven prosphora during Divine Liturgy instead of five. They also write Christ's name with a single I (Isus) rather than two (Iisus) and move clockwise around the church.
Archimandrite Spiridon argued that Nikon's changes constituted the Great Apostasy before the Day of Judgment in 1666, leading followers to believe the Russian church was ruled by Antichrist. This eschatological current became deeply ingrained in their worldview, shaping actions such as mass suicide between 1670 and 1682.
The division emerged following the death of pre-schism priests when Northern dissenters adopted the doctrine of the spiritual Antichrist in councils held in Novgorod in 1692 and 1694. Southern communities accepted banished clerics from the official church to continue full liturgy, while Northern groups passed leadership to laymen known as nastavnik or nastoyatel.
Over one million Old Believers reside primarily in Russia, Latvia, Lithuania, Romania, Ukraine, Belarus, Estonia, and the United States at the dawn of the 21st century. The largest organization is the priestly Russian Orthodox Old-Rite Church with headquarters in Rogozhskoye Cemetery in Moscow led since 2005 by Metropolitan Cornelius Titov.