Celebrim
Legend
According to Ed, 99.999999999% of Faerûn firmly believes in the deities and pays homage to whichever one is appropriate at any time, regardless of alignment.
While I appreciate Ed's recognition of the fundamental nature of polytheism (in polytheism, you don't worship a god, and all gods are worthy of worship regardless of what they express), the notion that real people would find the FR dieties to be worthy of worship strikes me as ... comical.
The Olympic pantheon was a very powerful set of myths over the human imagination, and contained a lot of interesting dieties ruling over really iconic and important aspects of peoples lives and had a rich and varied worship life. In terms of ethics based on heroic narrative, they are probably among the more successful polytheistic visions of all time.
None of that seems true for me for the Faerun pantheon, even the ones borrowed at least in name from real mythology (and there are a lot of those, especially originally). Yet even the Olympic pantheon, which overran and absorbed Romes, tended to promote a lot of skepticism and ennui. Socrates is probably the most famous example, for saying (in effect), "I believe that there must be Gods, but I also believe they can't possibly be anything like the stories we have about them." By the time of the Roman empire, most citizens are already agnostic and fed up with traditional religion with its self-absorbed comic book heroes and long lists of gods so banal that the stories about them had already been forgotten for centuries so that not even their priests could tell you why they were worthy of worship. Fundamentally the problem I have is I can't believe anyone actually living in the world is inspired to live by the example the gods set, which is a concept fundamental to polytheism particularly.
Examples of believable speculative polythistic inventions where you could believe that the world had a thriving religious life would be the The Rulers of Heaven from 'The Book of the Righteous' or the Five Gods from Bujold's Chalion series (especially as presented in the first book, later works somewhat undermine it IMO). Faerun doesn't cut it IME. I strongly suspect that 99% of Faerun's inhabitants just want the gods to go away.