After making allowances for countries that have, or recently have had, an officially imposed atheist ideology, in which there might be some social pressure to deny belief in God, one can venture conservative estimates of the number of unbelievers in the world today.

Reviewing a large number of studies among some fifty countries, Phil Zuckerman, a sociologist at Pitzer College, in Claremont, California, puts the figure at between five hundred million and seven hundred and fifty million. This excludes such highly populated places as Brazil, Iran, Indonesia, and Nigeria, for which information is lacking or patchy.