Entering the Clergy
As a bonus, you can ask the pope to make any one of your un-knighted NPCs a priest. This boosts Stature by 1 (useful for bailiffs.)
As a bonus to the bonus, you can also the petition the pope to make one of your priests an abbot, this bumps the Stature up another 2 (abbots often presided over huge estates.) You can, with the popes permission, make an unmarried female NPC an nun (stature boost of 1) and then, when she is over age 40, an Abbess and boost her stature by 2 (it's about the only break we can justify historically for the women.) An abbot can also be promoted to bishop, which bumps Stature another 2. Keep in mind that, while these clergy are part of a nobles household, the pope may hire them away at any time, without penalty. All clergy are answerable to the pope, even though they might, at the pope's sufferance, spend their entire lives in service to a lay lord. Clergy should not be used in warfare, although it did happen. Depends on how well you get on with the pope. Clergy may not marry.
For an NPC to enter the clergy the NPC must have Piety Extraordinary or Pious skills. These NPCs may not be used to lead armies or do dirty deeds. If they are, their current owner is excommunicated (if the pope does it, well, the lords of Christendom have to do something about it).
A nobles son may be admitted to Holy Orders without the needed skills at the popes discretion (you know what that means, so dig deep.) All clergy thus represented in the game should belong to the popes organization. These NPCs must be sent to the pope to be "hired."
Daughters may also become sisters and, once they are over age 40, may be promoted to Abbess. Her name is then changed to ABBESS FIRSTNAME and her Family Name to that of a Province in which she will reside (and preside over a rich Abbey). Seducing a nun is an excommunicable offense, as is any assault on the clergy.