The author explores the preparations for the Soviet 1937 census. (The results of this census were famously cancelled by the authorities for reasons of alleged falsification, and people involved were subsequently persecuted). In this census, for the first time in Soviet history, the question was included about the respondents’ religious affiliation. In this connection, a special reference book has been created that covered the full list of religions in the USSR. The reference book described in detail various religious groups; in fact, the researchers created a comprehensive “map” of religions existing on Soviet territory. This “map” allows us to get an idea of religious diversity in the mid-1920s., when the data was mostly collected. The Addendum to the paper contains the full “Systematic Index of Beliefs (Religions).”