In other words, in order for the Lord to confirm your prayer that the Book of Mormon is true, you need to firmly believe that it is true before you even ask.
Moroni also said it very clear here:
Moroni didn't say anything. Moroni was nothing more than a figment of Joseph Smith's vivid imagination.
6 But let him ask in faith, nothing wavering. For he that wavereth is like a wave of the sea driven with the wind and tossed.
7 For let not that man think that he shall receive any thing of the Lord.