Marx regarded religion as the "opium of the people" and a false belief that was used to convince them to obey the ruling elite under tsarism.
The Bolsheviks were determined to reduce the influence of organised religion over the people of Russia after 1917.
The Russian Orthodox Church was separated from the state in 1918, church land was confiscated, marriage was no longer controlled by the church and religious education was banned in 1921.
Propaganda discredited religion and the "Union of the Militant Godless" promoted atheism.
Christmas and Easter were replaced as national festivals by New Year and May Day.
During the Civil War, priests were deprived of rations and many died, church valuables were seized and priests and church leaders who uttered anti-Communist ideas were arrested.
Muslims suffered similar persecution as the property of mosques were seized and Sharia courts were abolished,
Under Stalin, Komsomol members were encouraged to increase the harassment of priests, the desecration of churches and the seizure of valuable objects.
In 1929, all congregations and their places of worship had to be registered with the government.
In 1932, the introduction of a 6 day week prevented a holy day of worship.
Stalin's 1936 Constitution made the publication of religious propaganda a criminal offence.
For Muslims, Mecca pilgrimages were banned in 1935 and wearing veils was forbidden.
During the Great Terror of 1936-38, senior priests and bishops in the Russian Orthodox Church were arrested.
By 1941, only 1 in 40 Orthodox Churches were still operating and the number of active priests was 9% of the figure from the early 1920s.