World of Origin's Age of Renewal
a) The Cleric is now a Prestige Class instead of a Class.
b) One can not decide to become a cleric. Instead it is solely up to the gods themselves to decide when they will bestow a portion of their power to a new individual, and whom it will be.
c) On the rare occasions when a faithful servant is chosen by it’s deity to become one of it’s clerics the god will personally give her a holy quest to fulfill in the deity’s name. If the candidate is successful, upon returning to her temple all her adept (and divine spellcasters) levels are instantly replaced with an equal number of cleric levels. As she sets foot inside the sacred area she becomes a vessel for her god’s divine power, and it’s voice among it’s people(s).
d) Usually only those who have risen among the ranks of the deity’s clergy have been chosen for this sacred task. This most often means advancing from adept all the way up to another clerc’s retinue of divine spellcaster’s. However, this may vary from deity to deity and the culture’s they rule over.