Theurgists combine and blur the magics of magic-users and clerics. They are devout
worshipers who are not granted spells through prayer, but who instead learn divine incantations and magics hinted at in liturgical texts. They are often in the roles of sages and liturgical experts, delving deeply into ancient texts searching for the gifts of magic.
[Author's Note: The theurge is very similar in powers and mechanics to the Bishop character class I posted for Swords & Wizardry - I wrote up the Theurge for my old B/X campaign, and adapted it for the Bishop class because I loved Wizardry so much.]
The prime requisites for theurges are both Intelligence and Wisdom. A theurge with a 13 in both Intelligence and Wisdom receives a +5% bonus on earned experience. A theurge with a 16 in both receives a +10% bonus.
Theurges can wear no armour and are restricted to the same weapons as a cleric, plus daggers. They cannot use shields. They attack in combat as a magic-user.
Theurges carry liturgical spell books, which hold the formulae and incantations for their spells written on their pages. A theurge can have any number of spells in their books, but can only memorize or prepare a certain number of spells at any time. Effectively they prepare and cast spells as a magic user, but use a special spell list of their own.
[Author's Note: Theurge spell progression is based on being a combination of magic user and cleric spell progression. If you are playing with a game that allows clerics to cast spells at level 1 (such as AD&D1e or Labyrinth Lord), increase the number of level 1 spells the theurge can cast at each level.]
Further, the divine knowledge of magic that theurges possess allows them to use any magic user and cleric scrolls, even if the spell in question is not on their spell lists. They can also use all magic-user and clerical magic items.
The Theurge (and nine other spell-casting classes for Labyrinth Lord) appears in Magical Theorems & Dark Pacts – a compendium of spellcasting classes and magics for the Labyrinth Lord game available in print and PDF from ZERO/barrier.