Swords&Wizardry Clerics have a property that is unusual in the D&D world - they do not get any spells as first level. They can still turn undead, but no spells until level 2, at which point they basically track the Magic User, but one level behind.
From a game mechanics perspective, I suspect that was originally done to balance out the Cleric's power level. After all, he can turn undead things at will, which is nearly a 1st level spell's worth of power. He also has a medium level of HP, can wear any armor, and has some nice weapon proficiencies.
But from a game world perspective, what does that mean? Clerical spells are powers granted by the divine, after all.
I've decided that what it means in my campaign is that Clerics have to earn their god's patronage by proving themselves. (That is, gaining a level) Once a cleric has leveled up, his exploits will be noticed by one of the lesser gods or godly servants, who will contact him and become his celestial contact.
A starting off cleric, while no doubt well versed in the faith he's pursuing, has yet to form a real bond with his god(s). Presumably, the cleric should perform acts that make the god he's trying to woo happy. However, it's also possible that the cleric will be contacted by a god other than his chosen god. This is likely in some mythoi, as the gods will compete with each other to some extent.
Also, in my system a character would in theory not need to be exclusive with his god, though he would treat with his god's enemies only at his own peril!
I'm a bit worried that my view of Cleric is bleeding over into Paladin territory, but those two classes are similar to begin with. I figure Paladins are introverted in the sense that they seek to do their god's bidding without question, but aren't showy about it. Clerics play the extrovert, making sure everyone is aware of the power of their god, and are more inclined to work with than for their god. (As in, "Mighty Odin, we can vanquish your foes if you grant us the boon of <insert spell list here>!" Whereas the Paladin would go out, vanquish the foes and shout, "Souls for Odin's Hall!" as he's killing enemies.) It's a fine line, and one which I don't think I fully grasp, not being a big fan of Paladins.