While I have not the slightest intention of becoming a Mormon or a Jehovah's Witness, I will say this much for those religions: They actually expect their followers to know something about the Bible. By contrast, most mainstream Christians appear to have a knowledge of their own holy book derived solely from Peanuts specials.
Nonetheless, knowing some proof texts is not the same thing as being able to think through one's faith. In my experience, if you ask adherents of those religions enough probing questions or even just let the them talk enough, they will reveal a contradiction in their belief systems. They truly do not appreciate having that contradiction pointed out to them.