What would it take for a significant portion of the population of the Balkans (from Slovenia down to Greece) to have at least a significant portion of the population turn Protestant (At least 10%, but aim for plurality/majority territory if possible).
If plausible, where would these Protestants most likely live? And what kind of Protestantism would be present in the population.
Edit: This is mainly out of a desire to see if adding yet another sectarian group could further screw the region.