...pulled from an article in an academic missionary journal. It suggested that the ratio of "evangelical" Christians to the rest of the world has been steadily increasing since 33 A.D. I'm going to totally make up numbers now to give you the feel, and try to look up the actual ones later--something like:

A.D. 40 - 1 in so many millions
A.D. 100 - 1 in thousands
A.D. 1000 - 1 in hundreds
A.D. 1800 - 1 in 50
1900 - 1 in 25
1950 - 1 in 15
1970 - 1 in 12
1976 - 1 in 9

I'm most sure about the last figure. Can't speak for methodological details at the moment--I'll try to find the source.