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.
So if you have faith in God, a sincere heart and real intent while praying about the Book of Mormon, the Holy Spirit will reveal that it is true. It almost sounds like Moroni is blaming you if you don't receive a confirmation of the Book of Mormon.
I was recently talking with some missionaries about this issue and they suggested I read Alma 32. It compares hearing the Book of Mormon to a seed and encourages us try an “experiment” of planting the seed. If the seed grows, then we know the Book of Mormon is true. But what if the seed doesn’t grow or produce fruit?
38 But if ye neglect the tree, and take no thought for its nourishment, behold it will not get any root; and when the heat of the sun cometh and scorcheth it, because it hath no root it withers away, and ye pluck it up and cast it out.
39 Now, this is not because the seed was not good, neither is it because the fruit thereof would not be desirable; but it is because your ground is barren, and ye will not nourish the tree, therefore ye cannot have the fruit thereof.
This passage actually does blame you if you don’t receive a confirmation. You could question this idea directly or take another approach. If this model of praying about a faith system is reliable, then wouldn’t it work on other faiths? So make a deal with your Mormon friend. Tell them you’ll agree to pray about the restored LDS Gospel if they pray about the preserved Gospel.
Paul made it clear in Galatians 1:8-9 he had already explained the Gospel to the church. But to be sure, he went over it again. He also talks extensively about the Gospel in Romans. Our claim is that careful, exegetical study of those texts, along with other supporting texts in the New Testament, is enough to believe in, follow and teach others about the full, complete Gospel of Jesus Christ.
Your Mormon friend may not see the truth of the preserved Gospel after the first prayer, but don’t let that discourage them. It just means they need to have more faith in God and the preserved Gospel. They just need to keep having faith in the preserved Gospel while praying about it until the truth is revealed.
They may say something about how absurd that is and they would be right. God may use a prayer like this to reveal truth to someone, but you can’t base a whole belief system on the prayer. But if they can’t trust this experiment to test the preserved Gospel, why do they put so much faith in it to test the restored Gospel?