The moment
In January 1918 the Bolshevik government decreed the separation of church from state and school from church: religious bodies lost the right to own property or to exist as legal persons, all church property became the property of the people, and religious teaching was banned from schools. Churches were closed and clergy arrested and shot in waves through the 1920s and 1930s. Stalin reversed course in wartime, receiving three metropolitans on 4 September 1943 and allowing a patriarch to be elected, but Khrushchev closed about half of the Orthodox churches still open from the late 1950s, and only a law of 1 October 1990 restored legal standing to religious organisations.
People: Patriarch Tikhon, Metropolitan Sergius, Joseph Stalin, Nikita Khrushchev