I'm not sure how it was handled in the 3rd edition, but nowadays the publications clearly state that the elves (and everybody else for that matter) believe in gods, because it would be foolish not to.
As I said, the gods are an observable force and everybody knows that they do indeed exist, and no one would deny their existence.
Everybody can see the forces that a priest can conjur (through liturgies and miracles), and sorcerers (which all elves naturally are) can determine through the use of magic that what priests are conjuring is indeed not magic. So what else could it be?
And then of course the gods (seldomly) actively intervene in worldly affairs, for everybody to see.
Then there are the countless divine artifacts on Ethra (in Aventuria we know most of them). Many of them clearly too powerful to be of magical origin:
- Rondra's chariot
Thunderstorm, drawn by four immortal steeds and still disappearing during every thunderstorm, only to reappear afterwards, dripping and glowing.
- Praios'
Eternal Light, a floating globe of pure light.
-
Hesinde's hopper, an alchemical instrument that can basically disassemble anything (anything yo can fit into it, that is) into its basic chemical elements.
- Travia's
Holy Cauldron which can feed a huge ammount of people from basically nothing.
- The list is endless...
And of course the gods play an important role in the history and the lore of the elves.
Etc.
There are just too many obvious facts, that undoubtedly proove the existence of the gods, to be able to deny their existence.
The Elves just chose not to worship any gods anymore (partly because of the bad experiences the high elves had with the worship of their gods - I would have to look that up to get into more detail).