Islam is currently fastest growing religion. What about irreligion?

[Angrboda]

Determining what percentage of a given society believes in God – or doesn’t -- is fraught with methodological hurdles. First: low response rates; most people do not respond to surveys, and response rates of lower than 50% cannot be generalized to the wider society. Secondly: non-random samples. If the sample is not randomly selected – i.e., every member of the given population has an equal chance of being chosen -- it is non-generalizable. Third: adverse political/cultural climates. In totalitarian countries where atheism is governmentally promulgated and risks are present for citizens viewed as disloyal, individuals will be reluctant to admit that they do believe in God. Conversely, in societies where religion is enforced by the government and risks are present for citizens viewed as non-believers, individuals will be reluctant to admit that they don’t believe in Allah, regardless of whether anonymity is “guaranteed.” Even in democratic societies without governmental coercion, individuals often feel that it is necessary to say that are religious, simply because such a response is socially desirable or culturally appropriate. For example, the designation “atheist” is stigmatized in many societies; even when people directly claim to not believe in God, they still eschew the self-designation of “atheist.” Greeley (2003) found that 41% of Norwegians, 48% of the French, and 54% of Czechs claimed to not believe in God, but only 10%, 19%, and 20% of those respondents self-identified as “atheist,” respectively. A final methodological problem: terminology. Definitions of specific words seldom translate well cross-culturally. Signifiers such as “religious” or “God” have different meanings in different cultures (Beyer, 2003), making cross-national comparisons of beliefs between markedly different societies tenuous.

Atheism: Contemporary Rates and Patterns : Phil Zuckerman