Related, a fascinating article.
BBC News - Religion may become extinct in nine nations, study says
The paper uses mathematical models to show that if being non-religiously affiliated has a higher perceived utility, which is defined as "a quantity encompassing many factors including the social, economic, political and security benefits derived from membership as well as spiritual or moral consonance with a group", then the number of religious affiliates will decline until there are virtually none left. This has more to do with group psychology and nothing to do with evolutionary arguments but I think those are also important. It will be interesting to see if group utility or evolution prove more powerful.