From the current CIA Factbook:<quoted text>
" follows these studies, though, I have to say that the 2012 figures are probably not from a poll that was conducted in 2012"
The United States "Central Intelligent Agency" (CIA) claims they took the survey in 2012. I'll take their word for it over your doubts.
Sorry you have all these issues with all the well established agency that took these surveys
"Field info displayed for all countries in alpha order. Christian 33.39%(of which Roman
Catholic 16.85%, Protestant 6.15%, Orthodox 3.96%, Anglican 1.26%), Muslim 22.74%, Hindu 13.8%, Buddhist 6.77%, Sikh 0.35%, Jewish 0.22%, Baha'i 0.11%, other religions 10.95%, non-religious 9.66%, atheists 2.01%(2010 est.)"
So the CIA actually says the estimate is from 2010, but they don't say how the it was made. Looking at the entries for individual countries, it appears that the CIA relies on local extra-agency sources for the proportions in each country and compiles the worldwide statistics from there. Some of those statistics show the hallmarks of estimates that are not based on actual studies. I get suspicious when the proportions are too neat like the 80%/19%/1% ratio seen for Afghanistan or the 50%/25%/24%/1% for Zimbabwe. The estimates for China are incredibly farfetched--Daoist (Taoist), Buddhist, Christian 3%-4%, Muslim 1%-2% note: officially atheist (2002 est.)--and account for only 4%-6% of the population. The ones for India are from a 2001 census and don't account for atheists at all, perhaps because theism is not necessarily a part of one of the major religions.
Given all of that, the estimate in the CIA Factbook seems to be less than precise.