A coworker of mine was recently given a religious pamphlet, which, out of a kind of morbid curiosity, I read voraciously. My favorite part was this hopelessly circular reasoning:
The first reason that we can believe what the Bible says, it would seem, is because the Bible tells us we can believe what the Bible says. Even though this was not an LDS pamphlet, I immediately thought of Moroni's Promise.
The way that we can know that the Book of Mormon is true is by following the Book of Mormon's own instructions for learning of its veracity. If you're wondering if someone is lying to you, asking him if he is won't help. If he was lying, he can just lie again. But if you talk to someone else who knows him well, maybe that person will have some insight that can help you make an informed decision.
The fewer resources you use in your quest for truth, the more likely it is that you'll be fooled. And with something so important as your eternal welfare supposedly at stake, it should be a priority to avoid being fooled.