Original: In the context of Russian Orthodox church history, the Old Believers (Starovery,) became separated after 1666 from the official Russian Orthodox Church as a protest against church reforms introduced by Patriarch Nikon between 1652-1666.
Simplified: In the context of Russian Orthodox church history, the Old Believers became separated after 1666 - 1667. They split from the hierarchy of the Russian Orthodox Church as a protest against church reforms introduced by Patriarch Nikon.