Before us, there was the old, cold world. The Creator broke from its bower in the West and passed overhead, first like a comet and then like a sun, and cast strong, bright light over what was.
There was Fire and Ix warmed her hands. "With this Fire," she thought, "I shall make great works."
When we say, "Fire" we do not mean, "fire." There are similarities: both are generative and also destructive. Both are insatiable and can burn out of control unless carefully tended. As there are those that worship gods of the hearth and those that worship the gods of wildfires, there are those that worship the Fire of life and those that worship the Fire of life unending.
When Magic Users spit "fire" spells they tap into the elemental plane of fire or shape the atmosphere, agitating it to extremes, just as when they spit cold spells they tap into the elemental plane of cold, call into being a bit of the old, cold world past or to come, or slow the atmosphere itself. The fire they call is not the Fire.
Magic is a gift of Fire, a byproduct of an exchange. By ritual and intent, the Magic User prods the Fire and by ritual and intent, a spits forth a spell: the Fire responds to the prod and a dribbled spark drops into the Magic User's head and rolls down through her mouth like an ember.
Fire is motion and life and the Magic User's mind explodes with vistas of wildly mutating landscapes. Magic use in this manner is addicting, with the Magic User either employing outrageous, dire, drastic means to retain control of their addiction or else falling head first into their addiction.
Magic Users know, from memory, Read Magic and Detect Magic. As a Magic User gains additional levels, they become stranger, their addiction to Fire growing stronger.
Clerics are granted miracles by the deities they worship. The minor deities are a multitudinous jumble and clerics that worship one, worship many.
The major gods feed on Fire to extend their lives and powers.
The will of the major gods is strong and most worshipers are left empty husks after years of adoration. The worshipper's essences leave their bodies and are taken up into the host of their god where they tend and feed the Fire. The templars and clerics of the major deities are mighty and powerful, more monster than man.