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http://en.wikipedia.org/wiki/Persecution_of_Christians_in_the_Soviet_Union
I quote from the article:
The history of Christianity in the Soviet Union was not limited to repression and secularization. Soviet policy toward religion was based on the ideology of Marxism-Leninism, which made atheism the official doctrine of the Soviet Union. Marxism-Leninism has consistently advocated the control, suppression, and, ultimately, the elimination of other religious beliefs.[1]
The state was committed to the destruction of religion[2][3], and to this effect it destroyed churches, mosques and temples, ridiculed, harassed and executed religious leaders, flooded the schools and media with atheistic propaganda, and generally promoted 'scientific atheism' as the truth that society should accept[4][5].
The Soviet regime was ostensibly committed to the complete annihilation of religious institutions and ideas [7]. Militant atheism was central to the ideology of the Communist Party of the Soviet Union[8] and a high priority of all Soviet leaders [3]. Communism required the abolition of religion [3]. Convinced atheists were considered to be more virtuous individuals than those of religious belief [3].
The state established atheism as the only scientific truth.[9][10][11][12][13][14][unreliable source?] Criticism of atheism or the state's anti-religious policies was forbidden and could lead to forced retirement, arrest and/or imprisonment.[15][16][17]
Some actions against Orthodox priests and believers along with execution included torture, being sent to prison camps, labour camps or mental hospitals.[24][25][26][27] Many Orthodox (along with peoples of other faiths) were also subjected to psychological punishment or torture and mind control experimentation in order to force them give up their religious convictions (see Punitive psychiatry in the Soviet Union).[25][26][28] During the first five years of Soviet power, the Bolsheviks executed 28 Russian Orthodox bishops and over 1,200 Russian Orthodox priests. Many others were imprisoned or exiled.[1]