Upvote:2
Here is one data point (from Jakob Burckardt's The Age of Constantine the Great) presumably among very many and already self-contradictory (translation courtesy of Google :)
The last time of Diocletian and Maximiam has come through the torture and blood streams of the great persecution of Christians into a horrible reputation. It has tried in vain to determine the amount thereof and the number of victims even close, so it has no basis each calculation, namely a reliable date on the number of existing at all at that time in the Roman Empire Christians. After Staudlin they accounted for half of the total population, according to Matter fifth, according to Gibbon merely one-twentieth, one-twelfth to La Basti, which perhaps comes closest to the truth.
As also indicated in my earlier comment, IMHO you can't hope for any definite answer to a question such as this one, the more so the "older" any conceivable numbers may get.