Sanctity

In GURPS...

Sanctity is the clerical magic's equivalence to mana level.

Power Investiture is clerical magic's equivalent to magery.

Every God will have Its own list of spells that are appropriate to them.  This is a tedious GM chore.

On the plus side, there's no prerequisite tree to follow and some of the spells can be very powerful.

On the down side, there are far fewer spells to choose from.

The ability to use these spells is subject to the capricious whim of mercurial Gods.

The major Gods: Aphrodite, Apollo, Ares, Artemis, Athena, Demeter, Dionysus, Hades, Hephaestus, Hera, Hermes, Hestia, Poseidon and Zeus can grant up to Power Investiture 5.

The primordial Gods: Aether, Ananke, Chaos, Chronos, Erebos, Eros, Gaia, Hemera, Hypnos, The Nesoi, Myx, Uranus, The Ourea, Phanes, Pontus, Tartarus, Thallassa, and Thanatos can grant up to Power Investiture 3.

The Titans: Asteria, Astraeus, Atlas, Aura, Clymene, Coeus, Crius, Cronus, Dione, Eos, Epimetheus, Eurybia, Eurynome, Helios, Hyperion, Iapetus, Lelantos, Leto, Menoetius, Metis, Mnemosyne, Oceanus, Ophion, Pallas, Persus, Prometheus, Phoebe, Rhea, Selene, Styx, Tethys, Theia, and Themis can grant up to Power Investiture 2.

Any and all other Greek deities (but not Gigantes or any Mortal, Hero or Deified Mortal) can grant up to Power Investiture 1.

The Fates: Atropos, Clotho and Lachesis, although very powerful, grant no powers or spells.

Did I mention that Magic Returns is a Greek Myth Parallel?

It's emphatically not Greece.

I'll be revisiting this with spell lists afore too long, but not anon.