Well, here's your problem right there.So, a divine caster like a Cleric, Paladin, Druid etc don't need any actual divine link to power and can just use "The power of my inner belief )or love or friendship or self respect etc) to gain divine powers.
More seriously:
I look at it this way. Some people have super powers (monks, psions, sorcerers) and some learn how to manipulate the "rules" (wizards) but everyone else needs either divine favor (paladins and clerics/druids) or to strike a deal (warlocks). I didn't mention rangers because they shouldn't have magic, and I didn't mention bards because they shouldn't exist.