Fallen Clerics in a Morally-Objective World

Waiting for the bus today, I began to puzzle over the idea of the corrupted D&D cleric and, by extension, the fallen paladin and the innocent church-goer deceived into cult worship. These things make good story hooks, but how could they ever happen? In D&D, deities grant spells directly, so how could a cleric ever unknowingly stray from the path of light? Surely the sudden lack of spells would be a wake-up call! Here's my take on it.

First of all, clerics can be one degree of alignment different than their deity. So a LG cleric of Heironeous could drift to LN without losing his spells. And a cleric doesn't have to be evil to disagree with the PCs and get in their way. Indeed, this can be an even thornier issue when the cleric is not evil and so cannot simply be dispatched.

However, the idea of a cleric actually turning to dark magic while still believing he serves the light is pretty thrilling. I think it could happen in D&D as follows: Most deities have many appellations beyond their normal (true) name. For example, Moradin of the dwarves is also called Soul Forger, Dwarffather, All-Father, and Creator. A cleric would likely develop their own personal appellation for their deity--even something as simple as "my Lord" or "my Light". Driven to distraction by sorrow, hate, greed, or a quest for revenge, they may rely on this personal appellation more and more as they slip from their true faith. Eventually, when they finally cross the alignment line and their deity refuses them spells, they would likely have a conflict of faith. Perhaps this would lead them to a period of fasting, flagellation, and praying, calling to their deity by their personal appellation. Eventually, their call is answered again. In their relief and rush to return to their work, they don't fully investigate the presence now granting their spells. (Indeed, there's probably a lot of self-deception going on by this point.) Unbeknownst to them, a new deceptive deity is now granting them spells and receiving their worship and appellation.

Now, when the PCs catch up with this cleric, they can still point out certain objective facts: the evil cleric channels negative energy, has an evil aura, and receives no reply if he calls upon his old deity by its true name. Faced with such facts, the cleric may be redeemable: he may atone and eventually return to the light. Or he may embrace, knowingly and whole-heartedly, his new patron deity and be forever corrupted to evil.