For Treasures, Serpents, & Ruins, I have five "holy" classes. Actually, there could be more, depending on the way you characterize them. This isn't counting Bards, Rangers, or Xia, who could all be played as adventuring religious figures if you so choose. But then, you could play any class that way, really.
Today, I'm focusing on these classes.
From TS&R Ruby: Cleric, Druid, Paladin
From TS&R Jade: Mudang, Sohei
Clerics are easy. Mine are more or less like the BX or BECMI class, except I use d8 hit dice and give them spells starting at 1st level, as in AD&D. They get blunt weapons, all armors, defensive/utility/healing magic, and can turn undead. Humans of course can be Clerics. I also allow Dwarves up to 8th level (an association from newer editions that I enjoy), and Half-Orcs can be Clerics up to 10th level. A bit higher than their AD&D level limit. What about Half-Elves, you say? Well, in my rules a "half-elf" is just a bit of roleplay you can add on to your Human or Elf character and has no mechanical effects. So "half-elf cleric" is just a Human Cleric with some angsty half-elf flavor.
Druids are a little more complicated, but not too much. I started with the template from the Companion Set, but allow them from 1st level. Also, taking a bit of inspiration from the AD&D class for them. They use a d6 for their hit die. They can only use organic or stone weapons, and wear only organic armor (although I have a few options besides leather, like silk/linothorax, or lacquered wooden lamellar), as in BECMI. They start with spells from level 1, and their spell list is similar but not identical to the Cleric list. I added a few spells from AD&D that aren't on the BECMI lists, and a couple of MU spells that are nature oriented, and replaced a few of the Cleric spells besides those "dealing with good or evil" as in BECMI.
For special abilities, Druids get Nature Lore: identify plants/animals, detect if food/water are safe to consume, and detect if plants/animals are sick or enchanted, each with a 1-4/d6 chance of success. They can also, like the Ranger, double the amount of food foraged by a party.
Next, they get their animal shape-change ability. I've been working to balance this for a few years now. Lately no one is playing a Druid in West Marches, but I hope this is the version of the power that works well: The Druid may take on the form of an animal for a total of 6 turns (1 hour) per day. In animal form, they keep their hit points, but all other stats are as the animal. At 1st level, they are limited to small harmless animals (sparrow, frog, mouse, beetle, etc.). Starting at 2nd level, they can transform into larger animals with hit dice equal to their level. So a riding horse, wolf, oil beetle or giant bat at 2nd level, a giant tarantula or black bear at 4th level, a triceratops at 11th level, and so on.
I think this gets the balance right. They can transform once for an hour, or six times a day for 1 turn each. They can't cast spells while in animal form. They can get access to things like poison bites from 2nd level, which is pretty big, but they still have limited HP and most animals don't have great AC. No where near as powerful as the 5E druid's shapechange which provides extra HP on top of the druid's own, and combat effectiveness from level 1.
Paladins are basically a Fighter/Cleric, as in AD&D, BECMI, and later editions. They get a d10 hit die, can use all weapons and armor, but have both STR and WIS as prime requisite scores. Their saving throw numbers are as a Cleric, but increasing every 3 levels along with their attack chances.
Special abilities, are unsurprisingly Lay on Hands, spells, Sweep, Courage, Multiple Attacks, and Dispel.
Lay on Hands, as in AD&D, grants them healing of 2hp/level each day, but only to other characters. I remember playing a Paladin in Pathfinder and only ever using LoH on myself. Not very knight-in-shinging-armor way of using the ability. At 2nd level, they can start casting Cleric spells, but they have a slower progression and cannot cast higher than 4th level spells.
Sweep is (as the SSI Gold Box Games told us) the ability to attack 1 creature of 1HD or less per level each round, as a Fighter.
At 4th level, Courage makes them immune to fear effects, and gives them a +2 bonus against charm effects.
At 8th level, they get 2 attacks per round when not using Sweep. They never get a 3rd attack, though.
Finally, at 9th level they can dispel magical effects on their own person 1/day.
Mudang, or shamans, are the TS&R Jade version of the Cleric. There are some important differences, though. They have a d8 HD like Clerics, and are limited to blunt weapons only (I may change this up a bit), but can only wear up to medium armors (chain mail, lamellar). They can use shields, too. They have spells from 1st level, and their spell list is similar but not identical to the Cleric spell list (spells from OA, Dragon Fist/Flying Swordsmen, and Chanbara add to the list). They do NOT turn undead. Instead, they get Resistance (+3 to a save 1/day/level) as in 1E OA (shukenja), but at 4th level I allow them to grant this bonus to an ally within 10' if they choose.
Next, they get a ritual ability. They spend 1 turn (10 minutes) performing the ritual and get one of these three effects:
* Trance: gain spirit vision (detect ethereal/astral creatures) for 1d6 hours
* Purify: sanctify an area (shrine, home, etc.) which wards out spirit creatures, undead, and demons.
* Soothe: allow up to 4 creatures affected by poison, disease, or other non-magical maladies to make a new saving throw against that effect.
Mudang can only perform 1 ritual per day.
Finally, they get 1d6 damage when using martial arts, which again is like the 1E shukenja or 3E shaman.
Sohei are a lot like the Paladin, in that they combine the Fighter and Mudang. They get a d10 hit die, all weapons & armor, but progress in attacks and saves as the Mudang and Cleric (ever 4 levels).
They get a combat Frenzy ability (bonus to hit, damage, AC, saves) for up to 1 turn. They can use this 1/day at 1st level, 2/day at 5th level, and 3/day at 10th level, with the bonuses equal to the number of times it can be used per day.
For spells, like the Paladin they start at 2nd level and can't gain higher than 4th level spells. They use the Mudang list.
At 2nd level they can Sweep like a Fighter, and at 8th level they get 2 attacks per round when not using Sweep, as the Paladin does.