[neohaven]

The one sin is to, as far as I can tell (not a Mormon here), refuse God's light once you have seen it.

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.

[tschwimmer]

Seems rather uncharitable. If you are not rational enough to recognize divine truth, are you really in a position to be condemned for your ignorance? Seems like it's targeted towards the mentally ill and disabled.

[rcxdude]

I think the point is it can only be committed if you understand it rationally and still reject it. So seeing the pearly gates and believing it to be a hallucination, for example, would not qualify.