In 1917 Tsarist Russia, the country was extremely socially and politically backward. Over 90% of its population were peasants who lived much as Russians in the 1700s had lived; isolated from urban life and therefore new ideas. They were extremely religious, with many believing the Tsar and his family to be appointed by God to rule; divine right was common among most monarchs in the centuries before.

The key here is that when the anti-religious Bolsheviks came to power, religious thought - while banned - never really went away throughout much of the Soviet Union's history. Urban workers might have been genuinely atheistic, but many peasants continued to practice religion in secret. Crucially, even Russians who discarded religion still had almost religious outlooks, with the cult of personality of Stalin portraying him as an omnipotent and omniscient leader. Throughout the 1970s until the collapse of the USSR in 1991, many Russians became extremely disillusioned with the regime and rejected party propaganda, and two key reactions of this during this time and beyond were expressed in nationalism and a revival of Orthodox Christianity.

Hope this helps.

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