I have been doing some research on world religions lately, and came across some summary data for the religious composition of countries. While these data are from a few years ago (for the most part), I decided to go ahead and add it to the world births/deaths simulation visualization - which I did today.
A screenshot after running it for a few seconds is shown below.
(Click here to go to site)
The data used include somewhat high-level categories (percentage of World religion in parentheses, per the Pew Foundation): Christian (31.5%), Muslim (23.2%), Unaffiliated (16.3%), Hindu (15%), Buddhist (7.1%), Folk Religion (5.9%), Other Religion (0.8%), and Jewish (0.2%). I also added the (interesting) Druze in Israel, as I just learned of them today (although it is unlikely to see anything happen in that case in a short run). The primary iconography for several of these is also included. On that note, I realize that using the cross for Christianity for both births and deaths causes a slight cognitive jar, and so this may be changed.
There seems to be quite a bit of additional data for some countries that will allow finer distinctions within religious groups (especially for Protestant groups within the US) - these to hopefully be included as time allows.