I recently convinced a secular Jewish friend of mine to read Sefer Koheleth. Like me, he quickly proclaimed it his favorite book in Tanach. But he also posed a question I couldn't answer: why is Koheleth part of the canon?
It's not particularly religious, and even touches upon heresy.
Some might argue that the last paragraph redeems it. Personally, the last paragraph has always seemed to me like it was tacked on by an editor. Still, even if one accepts the last paragraph as genuine, does it redeem the entire sefer? Why is it canon?