Speaking from my observations, believers in this category fall into a few subcategories:
1) Personal revelation. Often their particular experience is not revealed lest it be questioned, but they often admit that it is not useful for anyone other than themselves. The "why" of this experience is considered unknowable, and highly insulting if their rigor is questioned.
2) "The WORD is still alive." This subcategory, following the strong tradition of anthropomorphism, maintains that the divinely inspired parts of the Bible will make themselves known. In other words, the wheat will separate itself from the chaff as a conscious entity. There does not appear to be any explanation for this other than certain parts "resonate" more with the reader.
3) Peer pressure. This group was raised as Christians and their belief is based solely on that fact. Ultimately most justifications for their faith are based on incredulity that their upbringing was deeply flawed; it would be like someone pointing out that you don't *know* that your sister/brother is actually biologically related to your family at all.
4) True ignorance. Some people never think about it at all. This may be surprising for people who come to a forum like this, but large swaths of the population have never considered the question of religion at all. These should be distinguished from #3 because they are not incredulous of the possibility of being wrong, it just never occurred to them.
I have probably missed some, but we can add them as they come up.