Sinder Velvin: Talking about the Lessons of Vivec, why did you write Sermon Zero? Should it be interpreted as being official lore?
Douglas Goodall: I wrote it is as a kind of "me, too!" after reading the 36 Sermons. It was a tribute and a refutation.
I don't have any say anymore about whether it is official lore. I probably didn't leave extensive enough notes for them to make it official...
I figured that, regardless of whether the 36 Sermons were true or not (something that was not decided at Bethesda when I worked there), the author (whether it was really Vivec or not) would have competition. An opposing faction. An alternate take.
Note that Sermon Zero isn't actually present in Morrowind, as far as I remember. Books that are actually published in one of the Elder Scrolls games have precedence over ramblings on the forums.
Hint: The best place to hide something is in plain sight. I believe this hint also applies to the other Sermons.
Sinder Velvin: I understand that there is at least one secret message in Sermon Zero that has not yet been discovered by the fans - the third secret of the thrice-secret word. However, it is uncertain whether the fans will ever discover it, so could you tell us what the secret is? If not, could you give the community a hint (preferably not a very vague one, hehe)?
Douglas Goodall: The third secret isn't something else to decrypt. It's the meaning of the other messages (and of the sermon itself). As I said, the best place to hide something is in plain sight.
I wrote Sermon Zero in a few hours. I was in a hurry, so I stole the wording (though not the meaning) from the overly complicated Rennes-le-Chateau hoax. That might be a good place to start, as it will lead you to all kinds of nonsense, some of which will help you interpret my nonsense (and Kirkbride's nonsense). Assuming you have time to spend on nonsense.
This is a fragment of UESP's Douglas Goodall Interview.