Essentially there will be four 'religious' classes:
1. Cleric - standard PH- think adventuring priest.
2. Mystic - Based on the "Favored Soul" class, except I don't want to buy the book.
Mechanically, Has a limited spell list but can cast more spells per day than the cleric. Think Sorcerer class, but with divine spells.
I guess I think of this class as perhaps eschewing organized religion, Perhaps carrying Celestial blood, or specially blessed at birth.
3. Archivist - essentially a Wizard, but casting Divine Spells. Also has some other special abilities. Definately tied to organized religion - but a scholar, rather than an adventurer.
Its an official class - look here:
http://www.wizards.com/default.asp?x=dnd/ex/20051007a&page=3
4. Good old Paladin. Standard except I'm changing the warhorse rules.
If you play a Mystic/Cleric, it be similar to a Sorcerer/Wizard, you'd keep two distinct spell lists. It would be a bit difficult from a background perspective- but if you were creative you could pull it off.