After WWII Greco-Catholic religion was banned in Czechoslovakia and Ukraine. Priests were forced to convert to Russian Orthodox. Those that stayed true to their religion and oath to Christ were prosecuted, thrown into labor camps and jails.
“Russian Orthodox Church” was stuffed with KGB agent “priests”.
True believers went into underground and stayed true to Christ. Eastern European church became purified and came out stronger.
“The Killing Grounds” indeed.