The lord spiritual is the member of the council who acts as the ruler's personal advisor on matters of faith. It is not necessary for him or her to be a member of the clergy. The position of lord spiritual is available to those vassals and courtiers who are male, over 16 years old, not a prisoner, and not incapable. Pagans (apart from Aztec and Tengri) allow female priests. The lord spiritual must have the same religion as his or her liege. For Muslims, the lord spiritual must have a religious educational trait.
Lords spiritual are given a small monthly salary and a monthly prestige bonus, and upon appointment the lord spiritual will gain a +15 opinion boost towards his or her liege. Characters with a high learning stat are preferred for this position. Lords spiritual can be tasked to do three things: proselytize, reseach cultural tech, and improve religious relations (replaced with perform charity for Muslims).
Proselytize - the lord spiritual is sent to a particular county to proselytize there. There is a chance of converting the county as a whole, and a chance of converting a local noble. It is also possible a local noble will be revealed as a heretic. There is a small chance the lord spiritual will be attacked by heathens, potentially wounding or maiming him or her.
Research Cultural Tech - the lord spiritual is sent to a particular county to research cultural tech there. While the lord spiritual is in that county, the culture technology spread rate will be increased, with a higher learning stat yielding a greater increase. There is however a small chance the lord spiritual will be wounded or maimed by a heretic, or that a great philosopher will be discovered, further increasing the culture technology spread rate.
Improve Religious Relations - the lord spiritual is sent to a particular county to improve religious relations there. There is a chance the relations between the lord spiritual's liege and the local clergy will be improved, and a small chance the relations will be worsened. This is particularly good for improving relations with a religious head (e.g. the Pope). This action is not available to Muslims.
Perform Charity - the lord spiritual is sent to a particular county to perform charity there. While the lord spiritual is in that county, the local revolt risk will be decreased, with a higher learning stat yielding a greater decrease. There is a chance the local noble will be impressed, improving relations between the lord spiritual's liege and the noble, and there is a small chance the lord spiritual's liege will gain piety. There is a small chance the lord spiritual will be trampled by a mob, potentially wounding or maiming him. There is also a small chance the lord spiritual will pocket some of his liege's money. This actions is only available to Muslims.
The lord spiritual of a Catholic ruler is instead known as a Court Chaplain. Similarly, the lord spiritual of an Orthodox ruler is known as an Ecclesiarch, the lord spiritual of a Muslim ruler is known as a Court Imam, the lord spiritual of a Norse Pagan ruler is known as a Seer and the lord spiritual of a non-Norse Pagan ruler is known as a Diviner.