The mad mob does not ask how it could be better, only that it be different.
— Martin Luther
The very first level of the "holy line" line of chivalric order units, Religious Fanatics are available to any and all Christian factions, and can be used by factions who for various reasons are unable to attract Nobles' Court units. These are men from all walks of life - peasant, knight, merchant or priest - who have for varying reasons turned to the vocation of holy warriors and enforcers of their faith. Despite being armed with long staves, do not underestimate their skills at combat - they can easily despatch most Dark Age infantry units in several hits, were it not for the fact that they had no armour and very little hitpoints. What they are better at doing, however, is in inspiring your army to perform better in combat, increasing the offensive stats of any infantry or cavalry in their midst. Only Western factions that have accepted Monotheism can recruit these men, which unlike the more defensive Militant Clergy will happily follow your troops to the ends of the earth to spread the word of God and fear in the hearts of the heathen hordes!
If you are playing as a non-Christian faction, however, do not despair - the east has a variety of counters for these units. Chief of these are archers and horse archers. If your opponent is utilising Religious Fanatics, silence them with archer fire or horse archery - despite their cross-country performance, their lack of armour makes them highly vulnerable. Another method for the Papal States and the Muslims when facing these units is to send in specialised cloaked units to finish them off. Five Vaccari or a horde of Fedayeen can easily cut these units down.
Despite the pacific teachings of Christianity (Exodus 20:13; "Thou shalt not kill") it was clear that to survive in the chaos of Dark Age Europe, it was impossible to survive, unless some use of lethal force or a neutralisation of the same of an adversary was used to protect one's livelihood. As a result, it was not surprising for Christians to overlook this fact, and to adopt a world-view that was more akin to that of the New Testament. In the face of imminent death, destruction and damnation, it was not difficult for many who were threatened to take up arms in defence of church and country. The Crusades were the culmination of such religious sentiments, alongside the political and situation of Europe and the Middle East at that time.