Joshua Dyal said:
Both Zeus and Jupiter can be traced linguistically to (and I'm simplifying, since we don't have all the linguistic characters here on the boards) *dyaus-pitar, the primeval Indo-european "sky father." Thor from Norse mythology, Indra from Sanskrit texts and Perkun from Slavic mythology are also the same god as Zeus/Jupiter, and in many cases the Romans at least did recognize this fact. They are all based on the same Indo-european root god and the differences can be explained by regional developments after the split of the various proto-Indo-european peoples and cultural influences from other cultural groups.
Actually, that's something I wasn't aware of. Thanks!

(Learn something new every day here...) I have always heard the "absorption theory", even from college professors...
My use of the Greeks was to avoid the Romans' "God Borrowing," but the point is well made that Faerun share cosmopolitan similarities with Ancient Rome. In Rome, two major initial competitors to Christianity were the Cults of Mithras and Isis, respectively - the Romans did a LOT of borrowing, ignoring the issue of the Indo-european connections: One of the effective ways to mollify a conquered people is to show you respect their religion, and this is where a lot of different deities entered the Roman lexicon.
Too bad that the Grehawk method was not followed: Using the old classic Greyhawk materials, you can actually trace which god was from which culture, and the MIGRATIONS of the peoples that spread their gods' influences. You can tell which deities were Baklunish, Suel, Oeridian, Flan, etc.
For my homebrew campaign, the point is moot; I use only about 20 deities, borrowing the concept of Dragonlance directly. All peoples worship these 20 gods (or are atheistic), and each race knows them by different names. However, their alignments are so broad, they will take worshippers of many alignments, and each one acts as a "subcult" within the religion (much like the "bajllion" sub-branches of Christianity or Islam). Since each cleric can cast even low-level miracles on sheer faith alone in my campaign, no one has the 100% claim on the "correct" path of its faithful.