So in your games, if a Cleric completely renounces their deity, you know, like, "I, Foolharidious, renounce Zeus as a good-for-nothing who can't get his fat behind off of Olympus, and probably doesn't even exist," then he still retains his powers? Even at, say, level 20? Even, you know, divine intervention?
Re-reading the cleric entry, I noticed two things.
1) Repeatedly, the fluff talks about the deity choosing the cleric, not the other way around. TwoSix said it well:
Just because you're done with your deity, doesn't mean your deity is done with you.
It's quite possible that when you tell Zeus to stuff it,
Zeus knew all along that you were gonna say that, because, you know, he's a god and everything. There's a Divine Plan at work, and for whatever reason, giving you divine magic furthers Zeus's ethos. I mean, what if Zeus is the god of atheists?
Paladin of Souls by Lois McMaster Bujold is an excellent example of a "cleric" who doesn't seem particularly like her deity. (It's also just a really amazingly good novel. Please don't be put off by the word "paladin" in the title; it's a non-traditional fantasy setting so the title character is really not very paladinny at all.)
2) To prepare a new list of spells, you must spend time in prayer and meditation. To access Divine Intervention, you must call upon your deity. In the fluff text of the class, it mentions that "the ability to cast cleric spells relies on devotion and an intuitive sense of the deity's wishes."
Is your hypothetical PC still willing to do all those things (prayer, meditation, call upon your deity, etc.)? If so, then I'd say that your actions show your continued devotion, even if your flippant words towards Zeus do not. If your PC isn't willing do to those things, then they are deliberately forsaking the divine power that is granted unto them, and they won't be using those class features, either. So basically the words you say don't matter.
The tricky issue is "the ability to cast cleric spells relies on devotion" -- what if your other actions show lack of devotion? Like, what if Zeus is the god of law, and you start acting really chaotic? What if Zeus is the god of justice, and you begin slaughtering innocents? What if Zeus is the god of oaths, and you start breaking oaths left and right just to piss him off?
One could argue that these actions show insufficient devotion and cause clerical spellcasting to stop working. However, because the cleric also gets "an intuitive sense of the deity's wishes," the DM would have to tell the player that this is the consequence,
BEFORE the player took those actions. After all, if the fluff text about devotion is being treated as rules, then the fluff text about intuitive sense should also be treated as rules.