This is my issue with all religions. It is the word of G-d but travels thru man to the rest of us. Man is corrupt so that is a big problem.
And therein lies the faith aspect of religion. Otherwise, "a little birdie told me to write this", would be (and in fact is) the foundation of soem religions.
The way it was explained to me when i asked the question was as follows: Us Jews celebrate the holidays because our parents celebrated them back to time immemorial. Our parents are not liars so we inherently believe that they received this tradition going back to its origin. Therefore, the biblical narrative must be true.
If that is the case then we celebrate Shavuos, which memorializes the receipt of the Torah at Sinai. So part of the narrative that Moses ascended to Heaven and came down with the Tablets is fact. Then what part of Judaism requires faith?
The part that has him writing down everything else besides what he came down with.
Its oversimplified but I hope it helps.