Of course, this presumption of religious axioms also has a history of upsetting people of other religions, and the extensive information-gathering involved often uses borderline unethical methods, such as the various geneology research service websites that are run in one way or another by the LDS Church and include terms of service that let them use the info you piece together however they like.
(Note that despite commenting on this thread and being a Mormon I'm hardly an examplary Mormon in a variety of ways. It's rare that a topic comes up on HN that I know much about though, so may as well contribute some info in the spirit of satisfying intellectual curiosity!)
It would take a crazy man, the kind that would say "the sun does not shine when he sees it", to deny a revelation.
Arguably... Anyone who doesn't believe because they have not seen anything has not sinned. The sin would be, for example, to realize "the Mormons were right!" as you face the gates too paradise and say "I don't believe this. I am an Atheist." As you are staring down the pearly gates.
Basically, knowing you do the wrong things, since you ignore the truth in front of you, and doing it anyway. But you have to see it to really unforgivably sin.
It seems pretty well-specified. It's about internal peace, not external vision.