Martin's _Atheism: A Philosophical Justification_:
"The number of nonbelievers in the world today  is surprisingly
large. The _World Christian Encyclopedia_, perhaps the most comprehensive
source of religious statistics available, estimated in 1982 that by 1985
there would be about 210 million atheists and 805 million agnostics in the
world. This means that about 21 percent of the world's poulation consists of
nonbelievers. Moreover, this figure does not include members of religions
such as Jainism that involve no belief in God. Since this calculation is
based in part on public opinion polls and on governmental, church, and
business statistics, as well as on other data, there is good reason,
furthermore, to suppose that the actual number of nonbelievers is much
greater. In many countries there is social pressure against expression of
nonbelief in God. In addition, the yes-no format of typical surveys is
likely to result in the recording of responses of unbelief only by hardened
atheists." (p. 3).
Furthermore, of those who report that religion is "very important to
them", on further questioning many would likely reveal that they do not
believe in the literal interpretation of the bible, or possibly even the
existence of God or the divinity of Christ.
Unfortunately, getting such admissions is often like pulling teeth, and
simple "yes/no" surveys don't catch the full truth.