If the Cleric is the one choosing to lose his faith,
there are still Cleric Of Philosophies that have
your pre-existing Domains.
Or the specific Deity could belong to a Pantheon,
kinda like Eberron, so just because you're a little
waffle-y about Thor doesn't mean Odin suddenly
thinks you're an idiot worth abandoning.
The other side of it ... you might think you're still
worshiping your Deity, but some iffy malevolent
Power is toying with you ...