Moroni's promise is nothing more than begging the question.

Moroni 10

4 And when ye shall receive these things, I would exhort you that ye would ask God, the Eternal Father, in the name of Christ, if these things are not true; and if ye shall ask with a sincere heart, with real intent, having faith in Christ, he will manifest the truth of it unto you, by the power of the Holy Ghost.
5 And by the power of the Holy Ghost ye may know the truth of all things.

So they want us to find the truth through the manifestations of the Holy Ghost. Well, just how do we know that the manifestations of the Holy Ghost are really a valid measure of anything? This does not even address the issue of how do we know when we have had a true manifestation from the Holy Ghost. That is a huge problem unto itself.

Like so many things in religion, apropos of nothing some claim is made with no supporting rationale. Then a nice circle is made back to the original unsupported claim. This is exactly what happens in verse five above. How do we know? The illogical answer...because we do. I feel sorry for missionaries, and other sincere teachers, who stumble around trying to defend such illogical claims from direct questioning.

And thank you for giving an example of "begging the question." I a lot of people simply think it means the same thing as "raising the question." But the latter, of course, is not a logical fallacy.

Ask if these things are NOT true ?