Now one could explain druids, rangers, and maybe even paladins having access to divine magic without the presence of gods, but does anybody remember the old Lankhmar setting that had white and black magic. (divine and arcane respectively) Lankhmar had gods and the white mage could cast as a cleric, but they were unconnected. If you like, you could possibly still have clerics as they are, but explain differently where their magic comes from and not have any gods.