Bishops learn Mage and Priest at the same time, but very, very slowly.
Class: Dispell. Ghost/Demon enemies are destroyed without giving XP.
Class: Identify. Items are IDed. To ID the highest level gear from the bonus post-game dungeons with good success, you need a level 200+ Bishop.
Mastery: Mage spell damage up (Max 562.4%, weaker than Mages)
Mastery: Priest spell power up (Max 843.6%, weaker than Clerics)
Special EX Skill: 100 Knowledge
Starting Class: B. They learn magic really slowly. The Japanese wiki suggests that the XP to learn all the spells as a bishop is equal to the XP required to learn all the mage spells as a mage + the XP required to learn all the priest spells as a priest... times TWO.
Ending Class: A. They're the only class that gets ID and 9 casts of both Mage and Priest magic. Going Alchemist -> Bishop is a great way to make an endgame caster.
No Class Change: B. Innocent Gear is ok.