Most Popular Religion

In regards to:

Say Euty, would you happen to know of a place where rough statistics further back in time can be retrieved? I am curious now as to who has been winning the meme wars. Perhaps with some percentages of world population I could graph it out, or work up a stochastic matrix.

X% areligious
X% percent various animistic
X% various Christian (or greater breakdown even)
X% Hindu, etc

with say, a generation or two between samplings.