The demi-human classes have already been discussed in a previous post, so this post will deal with the remaining four classes.
The Cleric in this setting is typically, (and not too surprisingly) a Witchfinder, e.g. a man of the cloth who’s purpose in life is to seek out and destroy witches, warlocks, demons, vampires, etc. The Van Helsing type characters of old Hammer movies make for excellent clerics. Of course, a cleric might also be a more traditional priest or monk, if the player so desires.
The Fighter is pretty much what it is in other settings, e.g. someone proficient in fighting and warfare. What might be of importance in this setting is a fighter’s allegiance, that is, on who’s side is he or has he been fighting, and does he still hold some allegiances to one of the parties of the civil war.
The big question mark in a campaign seemingly geared towards hunting witches to extinction would obviously be the Magic-User class. The Magic-User taps the raw forces of chaos, harnessing the building blocks of the universe. Thus, as per the LotFP rules, all Magic-Users are of Chaotic alignment. The method of harnessing, however, differs from individual Magic-User to other. There are those who seek out otherwordly beings, and perhaps even worships them. These would be the classical devil-worshiping witches and warlocks. There are those who commune with forces other than the Christian god, and find power in places, objects, and creatures both mundane and otherwordly. There are the scientists and doctors who meddle with alchemy, and sciences most people today would consider ludicrous.
Any of these types of Magic-Users risk the wrath of superstitious mobs or zealous Witchfinders, and so they are forced to practise their respective crafts in secret, behind closed doors, or far away from population centers.
Needless to say, a zealous Witchfinder, and a Magic-User who openly flaunts his talent in the same player character party is an ignited powder-keg waiting to explode, and probably best avoided altogether. The same goes for the Elf.
The Specialist can be pretty much anything. The class is customizable enough to emulate any kind of, well, specialist, be it a thief, a grave robber, a scout, a spy, an assassin, a scribe, an interpreter, a wilderness guide, or what have you. The Specialist is what the player makes of him.