Forget whatever else you've heard. The only real difference between a cleric and a warlock is that one has an employer and the other has a sugar daddy.

A Theurge gains additional abilities in a flexible manner from a powerful, possibly supernatural source (such as a god, spirit, genius loci, devil, ancestral ghost, or whatever) in exchange for behaving in specific ways. A Theurge's patron of choice will have a tremendous effect on the way the archetype feels and plays. Creating a new patron is relatively easy, allowing the archetype to be customized to any setting- just run your ideas past the referee first.

Theurge Draw upon a higher power.
caret-right Bless Grant temporary boons to others.
caret-right Henotheist Access more patrons.
caret-right Pact Free boons and taboo forgiveness.
caret-right Petitioner Exchange boons on demand.
caret-right Veneration Two boons.

Theurge
Utility plus-circleIn Thy Name: You are devoted to a powerful supernatural being called a “patron” who grants you power in exchange for your worship and the furthering of its agenda. You have to pick a single patron when you take your first level in Theurge and can't change it later (at least not without a significant crisis of faith). Ask your referee what patrons are available; maybe they'll let you come up with your own if you want. Every patron has a portfolio, three observances, and a taboo. Some examples are below.

plus-circleBoons: If you manage to accomplish any of your patron's three observances during a session, they grant you a boon immediately afterward. A patron's boon allows you to temporarily gain the use of one archetype from their portfolio until the end of the current session. If you already have a level in an archetype granted by your patron, you can choose an advanced ability from that archetype instead if you want.

minus-circleOne Boon Limit: You can only have one boon active at a time, no matter how many times you fulfill your patron's observances. You can change which archetype/ability is granted by your boon every time you fulfill an observance.

minus-circleApostasy: If you ever break your patron's taboo, you immediately lose any boon they granted you and cannot regain their favor again for the rest of the current session. If you broke taboo in a very deliberate or outrageous way (referee's call) you might lose the ability to ever call upon that patron again permanently. If this happens, it's time to go on a quest to either find atonement or to find a new patron.

Example Patrons

Saint Regulati, Holy Slayer
Her mighty warhammer flashes like the sun, a weapon both symbolic and literal against the creatures of the night.
Portfolio Basher, Helsing, Steadfast
Observances Smash the shit out of something vile.
Give help to someone who cannot help themselves.
Face great danger head-on.
Taboo Allow a monster to live.
Confideus, Platonic Dad
He is not a patriarch deity. He is the deity of being a dad. There is a difference.
Portfolio Instructor, Outfitter, Vigilant
Observances Maintain or repair something.
Nurture somebody younger than yourself.
Make a really lame joke. Like, really lame.
Taboo Harm a child, or allow one to be harmed.
The Patient One, Keeper of Secrets
It waits for the end of the universe. This is not the first one it has seen.
Portfolio Oracle, Scholar, Striker
Observances Learn a secret.
Destroy something beautiful.
Mutate.
Taboo Knowingly tell a direct lie.
Gandukk, Ancestral Totem
Guardian spirit and iconic representation of the Red Bear tribe. He remembers when his chosen people were strong, and aims to guide them to glory again.
Portfolio Berserker, Hex, Tactician
Observances Destroy an enemy of the tribe.
Catch and eat a fish.
Get revenge for something.
Taboo Refuse help to any member of the tribe.
Brandish, Trans-Temporal Probe AI
Artificial mind in a starmetal body sent from the future. In a billion years its creators will dig it up, and it will tell them everything.
Portfolio Adamant, Chronomancer, Raygunner
Observances Bathe in the ocean.
Kill a mammal. Any will do.
Carve words in stone or clay.
Taboo Perform or enjoy music.
Cabbage, Crossroads Devil
A specialist in wishes for fame and fortune. Payment will be discussed later, just go have fun for now okay?
Portfolio Luminary, Merchant, Negotiator
Observances Gloat.
Flaunt your wealth/status.
Betray someone's trust.
Taboo Any form of charity.
Moranna, Veiled Sorrow
In the darkness of the underworld, she weeps and cradles millions of souls tight in her skeletal arms. They have suffered in the cruel world of the living, and she wants to help.
Portfolio Healer, Necromancer, Undead
Observances Mercy-kill. If you can save them another way, it's not mercy.
Share resources with another.
Reanimate the willing dead.
Taboo Eat meat.

Bless
Utility plus-circleFeel The Power: As an action, you can give your patron's blessing to any single willing target within 20 meters that you specify (including yourself, if you want). This grants your target a temporary boon from your patron of their choice that lasts until the end of the current exploration turn.

minus-circleInflexible: Only one divine boon can be had by any creature at a time, either from the Theurge's normal ability or from being blessed. Those who gain boons from blessings can't change them out for different boons by fulfilling an observance the way you can; what they have is what they get.

minus-circleMy House, My Rules: If you bless a creature who later breaks your patron's taboo, they lose the boon just as you would. Anybody who is a habitual breaker of your patron's taboo cannot be blessed by them in the first place no matter how many times you try.

minus-circleMojo: Blessings carry a tithing price. Every time you bless a target with this ability, roll a trigger die. If the result is 1-6, then doing so cost 1 Mojo.

Henotheist
Utility plus-circleDivine Contractor: You may obtain boons from patrons other than your normal one by fulfilling their observances.

minus-circleSingle Favor: You still may only have one active boon at a time. The Henotheist ability simply expands your available list of choices.

minus-circleNormal Disfavor: Breaking the taboo of a patron removes all boons they might be granting you as normal. Flagrant past violation of a patron's taboo might preclude them from granting you any boons at all, even with the Henotheist ability.

minus-circleContact: Other patrons aren't watching you all the time like your own is. You may only get a boon from a different patron while in their direct presence or while in one of their holy sites/shrines.

minus-circleJealousy: You cannot gain a boon from a patron that your normal patron strongly disapproves of. For example, Confideus from the example patron list above can find plenty of common ground with both Regulati and Moranna and wouldn't mind if you got a boon from either, but Regulati and Moranna are too different from each other to accept their Theurges getting help from the other. If you're not sure if two patrons could play nicely with each other, the referee makes the call.

Pact
Utility plus-circleYa #1 Boy: You're so devout/likeable that your patron no longer requires regular observances to give you boons. You start each new session already having a boon in place of your choice. You can change the archetype/ability granted by your boon later on by performing an observance as usual.
Utility plus-circleForgiveness: If you break your patron's taboo by accident or if there are reasonable extenuating circumstances (referee's call) then your patron does not take their boon back or get mad at you for it. Purposefully and knowingly breaking taboo still earns their wrath as normal.

Petitioner
Utility plus-circleGrant Me Something Else: You can choose to change your active boon archetype/ability to any other archetype/ability available from your patron as a procedure. You do not need to fulfill any of your patron's observances in order to change boons (although you still need to fulfill them in order to get a boon to begin with).
caret-right Alternative plus-circleOrison: You can choose to change your granted boon to a different one as a single action instead of taking a whole procedure.

minus-circleDivine Toll: Rapidly changing your granted boon in this way requires you to spend 1 Mojo.

Veneration
Utility plus-circleMost Blessed: Whenever you receive a boon from your patron, you can choose to receive two instead. This supercedes the normal rule that nobody can ever have more than one boon at a time; your limit is two.