Stalin’s parents were Georgian Orthodox, but he decided to be an atheist.
Stalin and his government did everything to raise an atheist generation; this included anti-religious propaganda at school, work and other social institutions; and severe punishment for all believers, conducted at the law level. By the late 1930’s people were afraid to publicly confess their connection with religion and church due to the risks of their health and life.
All these measures of oppression, Stalin kept implementing, led that Russian Orthodox Church almost disappeared as a public institution.
At the beginning of 1940’s the amount of actively working parishes were about several hundred (in comparison to 54 000 in 1917). Stalin’s politic led to the demolition of plenty of both small churches and big cathedrals; and to the killing or tortures of more than 10 000 church supporters during purges of 1937-1938.
World War II gave a little mitigation to the religion life as church was expected to serve as an organization to gather people around it. That is why many small parishes were renovated all over the country. This, however, didn’t last for a long time, as Khruschev’s rule continued the politic of the church oppression.
Because of Stalin’s and his government Russian Orthodox Church Synod’s approval, the Church outside the USSR split in two.
Situation for other religions, existed on the territory of the Soviet Union, was not better. Catholics, Jews, Buddhists, Islamists, Baptists and others were haunted and plenty of their sacral buildings were destroyed.