Oh, in the right situation, you can. I've killed about 90% of the population of a region in one go -- get your followers to rise up when they can't possibly succeed, and most get killed. If you have 99% followers and good oratory, then. . . .
If you have little following, then yours will die, and the others will remain. If you have strong following, then yours will survive, and the others will die. If you have as much following as dissent, then a part of both will die, but I have never seen proportions to indicate this could lead to 50% casualties in one go. It tended to result in more followers turning pagans then heretics being slain.
