Russia is undertaking a census of its people, sending about 650,000 workers out to visit every home in the country.
Population has been declining in Russia for years, though Prime Minister Vladimir Putin announced last year that the trend had been reversed.
Demographers believe there are about 140 million people in Russia.
The BBC reports the government says the data will be used for developing social programmes and for budget purposes.
But opposition parties say they fear what the government will do with the information. Some have called for a boycott.
The Orthodox Church is infuriated by the absence in the questionnaire of a question on faith.
A spokesman for the Russian Patriarch said: "They are afraid of knowing the state of religion in our society."