Boyaryna Morozova by Vasily Surikov (1884-1887), depicting
the martyrdom of Feodosia Morozova. Click to enlarge.
The Russian Old Believers arose out of the "modernizing" reforms of the Moscow Patriarch Nikon in the 17th Century. Refusing to buckle despite often ferocious persecution, they still exist today, albeit in a range of observance which runs from a hierarchy with seven sacraments to groups that deny there are any sacraments other than baptism. There are some in Michigan, and one of my wife's students was an Old Believer.
Say whatever else you will, the Old Believers had courage to spare, and as a result, they're still here.
Needless to say, I'm interested in books about them, so recommendations are more than welcome...
Right now, my Much Better Half is doing this: