This map has been doing the rounds:
Explanation at the original site:
Not religious, in this sense, is everyone who is left over when you have counted adherents. The World Christian Database, on which the data is based, use the term ‘nonreligionists’ and defines it as ‘ … encompassing the 2 varieties of unbeliever: (a) agnostics or secularists or materialists, who are nonreligious but not hostile to religion, and (b) atheists or anti-religious/anti-religionists opposed or hostile to religion.’
Territory size shows the proportion of people who are not religious living there. [Absolute numbers seem not to be a factor.]
Even after this gloss, ‘not religious’ is a dubious category, which tends to be defined in terms of Western (Abrahamic-Theistic) norms. (Is Chinese medicine, for example, in any serious way ‘non-religious’?) Nevertheless, the distribution is striking. Any conception of China’s rise (and it’s probable world cultural impact) is going to be missing something important if it neglects what is really jutting through here.