Caveat: This might be the wrong place entirely for this thread, and I also might be missing something laughably big as in I'll laugh when it is pointed out. Here goes.
To TLDR: I'm musing on a lightly armored caster flavor with the same kind of restriction and similar but not necessarily equivalent benefits that a paladin gets. What would be thematic and appropriate for something like that, what benefits would be cool and not overpowered? I would like this to be Pathfinder and Golarion related and that's why it is posted here but I understand if this is somehow not the right place.
Earlier today I was idly musing over ephemeral character ideas, and I happened upon this thought. The paladin is a lawful good martial character who follows a code of divine providence, to most gods that fall within one step of lawful good.
In return for a strict and staunch adherence to the precepts they've vowed to uphold, Paladins are granted divine powers to act as a beacon of goodness and law.
Now, there are classes that are casters with divine options. Clerics are obviously divine casters with obligations to their gods. Celestial sorcerers have divine bloodlines. Mystic theurges traffic in divine power at the same time as arcane power. Witches can have benevolent patrons.
However, none of these classes are required to actually adhere to a paladin-style code, where the divine aspect of their powers can be taken away if they fall, The cleric being the most similar exception. The fact stands though that they tend to utilize medium armor and weaponry of whatever kind is counter to the concept I'm trying to produce. They also don't have to hold to a specific code, just not offend their deity.
In essence I'm idly trying to create a lightly armored caster who carries within them the same quantity of discipline and classification of devotion a paladin does, who uses magic as their weapon rather than a sword, mace, or even a bow/crossbow/gun. I'm trying to think of what it would take to make that kind of character a presence akin to a paladin and cool to play.
My general thinking is that the baseline is any kind of nondivine magic as long as it isn't evil in alignment, and that similarly to the paladin they get some sort of boost atop that. Figuring out how to tune and flavor that in an appropriate way is the purpose of the thread.
Actually this is built into the cleric class and even to the inquisitor to a lesser extent.
Ex-Clerics
A cleric who grossly violates the code of conduct required by her god loses all spells and class features, except for armor and shield proficiencies and proficiency with simple weapons. She cannot thereafter gain levels as a cleric of that god until she atones for her deeds (see the atonement spell description).
The cleric class does not have a specific code of conduct required by all clerics, because each deity has a different code. Clerics can also be of any alignment so it would be impossible for a single code to cover all alignments. But clerics do have to follow the code of their deity.
I really like the Shelyn idea, Sarabel.
Also Mysterious Stranger, I'm not quite as familiar with inquisitor requirements. For clerics isn't that a general standard of behavior rather than a professed vow like I believe paladins have to make? I do believe that paladins aren't required to follow a specific god, but can, though. They do have to swear themselves to something, right?
Back in 3.X there was the Shugenja class, a slightly different kind of wizard, that utilized taboos, odd restrictions to the character's behavior. I don't remember it all that well but I recall examples of 'never cutting your hair' and such things. I realize this isn't exactly the same as what you're asking for but it might help you find some inspiration.
Inquisitors are allowed a lot more leeway than clerics. They must be within one step of their deity’s alignment. It specifically states they are above many of the normal rules and conventions of the church. Again it does not specify what slipping into corruption is because it will vary from deity to deity.
Ex-Inquisitors
An inquisitor who slips into corruption or changes to a prohibited alignment loses all spells and the judgment ability. She cannot thereafter gain levels as an inquisitor until she atones (see the atonement spell description).
Clerics and warpriests do have a code of conduct they are required to follow. Since any and all alignments have members of these classes you really can’t have an all-encompassing code like the paladins. How formal or informal these codes are will depend on the alignment and nature of the individual deity. The paladin’s code works because of the alignment restrictions of the class.
Lawful deities will have more defined codes than chaotic deities. Good deities will probably also be slightly more likely to define acceptable behavior. So a lawful good deity will probably have a pretty strict and formal code, while a chaotic evil one will have a very informal and lose set of strictures on behavior.
The Lawful deities will also tend to be quite formal including things like professed vows, and ceremonies granting certain rights or abilities. This is more a matter of role playing than of game mechanics. In game I run, if the deity in question has a paladin’s code all worshiper are required to follow it. So a cleric of Sarenrae for example has to follow the paladin’s code for Sarenrae. They do not however have to follow the standard parts of the paladin’s code unless it fits the individual deity. This seems like a lot of work but it is really just basic role playing.
What you are really looking for is the Champion of the Faith archetype for warpreists. They have less martial powers and more magic powers than paladins, but otherwise are very similar.