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