[MENTION=17077]Falling Icicle[/MENTION] When Druids choose to venerate "nature", the deity with the Nature domain is granting their spells.
Clerics don't get to choose Domains as their deities, because all domains have deities. They choose the deity with the domains (and therefor, "causes") that work for them. Then, they champion those causes (Sun, Good, killing undead, hey Pelor, how you doin'). The deity gives them their spells because they are doing the God's work.
That is true in some settings, like the Forgotten Realms, but it isn't true for every setting. The 3rd edition rules, by default, allowed clerics and druids to get their power from abstract sources instead of gods. The FR book explicity said that such wasn't the case in that setting. In the Dragonlance setting, on the other hand, there is also what is called Mysticism, a way of using divine magic without the gods, drawing upon the divine power of one's own spirit. Mystics in that setting are basically the divine equivalent of sorcerers.
Being granted spells by a diety is only one of many ways that divine magic could work. You make that sound like that is the only way, as if it were black and white, cut and dry, my way or no way. In your game world, maybe that is the only way divine magic works. But don't tell me that it has to work that way in mine.
At no point has it been suggested that "good", "evil", "darkness", or "the sun" are divine forces. It makes no sense at ALL for that to be the case.
Why not? Many cultures believe that things like the Earth, the sun, the moon, and other forces are divine. Some associate those things with gods or spirits, others simply see them as forces of nature, but still divine and deserving of respect and adoration. Maybe the gods themselves are just pawns of those same cosmic forces. Maybe the gods don't define good and evil, law and chaos, light and darkness, maybe those things define
them.
There is absolutely no difference in spirituality between a devout Fighter and a devout Cleric. They both went to Church, they both studied their deity, they both believe fervently, and worship every day. The difference is that the Cleric learned how to channel divine magic and the Fighter put his efforts into swinging a sword better.
And just what is the cleric learning when he learns "how to channel divine magic?" Is he studying a book and learning religious lore and rituals? Is that all there is to it? Or is it a spiritual journey? Is there a much deeper personal enlightenment required to work divine magic than simply uttering the right incantations?
There is no right or wrong answer to that question. I was simply offering alternative explanations that people can use in their games if they wish.
Everyone can pick up a sword and swing it to various degrees of success. Very few can channel divine magic.
If "championing a cause" with enough "spirituality" is all that matters, then everyone can channel divine magic to various degrees of success.
Anyone can pick up a sword, but that doesn't mean anyone can fight like a fighter. Anyone can pray and worship, but that doesn't mean anyone can use the miracle of divine magic. The way I see it, members of every class have gone above and beyond what ordinary people do, and are something special.