Question: I know there is a fallacy in this, but I can't spot it
If an event is not corroborated by evidence, reliable witnesses, or a pattern of similar experiences that are corroborated, then belief in it might well be blind faith, and we would rightly be suspicious. Not all faith is blind faith, however. We all have faith in gravity, because it is well corroborated.
A holy book isn't necessarily wrong just because it is considered holy or concerns matters out of the ordinary experience. The key is whether it can be corroborated. The Bible is well corroborated by evidence, by witnesses, and by a pattern of like-corroborated events. The Book of Mormon, on the other hand, has no corroboration.