Right now, levels 18 and 19 are dead levels. The capstone seems uninspiring as well. I have trouble seeing anyone interested in mechanical advantages taking this class past level 17. (someone posted before me, but it's important enough to repeat)
Also, I would suggest giving priests weapon proficiency in their diety's weapon, or weapon focus if their diety's weapon is simple. Any D&D player who comes into your system doesn't want to be useless at defending themselves, and it seems like you will have few strictly offensive spells for the class, meaning if a baddie closes with them, they are screwed. I'm not saying make them full melee, but at least let them defend themselves.
Also, your spell progression is rather stunted towards higher levels. Don't increase the uses per day all at once, spread them out. Make it look more like the sorcerer spells-cast-per-day (although obviously not as many spells per day)