We believe it for many good reasons, and this is not the place to detail all of those.
You were just asked how you know the Bible is absolute truth. It sounds like the perfect time and place to detail your reasons.
But the Bible has been researched more than any other ancient book, and surviving manuscripts have been compared to see what, if any, changes have occurred. The Dead Sea Scrolls included copies of parts of the Old Testament considerably older than any other extant copies, and these showed that very little had changed (and nothing of significance).
It is indicated several times in the Bible, frequently by God Himself, that not one "jot or tittle" (dot on the "i" and cross on the "t") should ever be changed. In fact God promises some dire consequences for anyone who would change even the smallest part of His word. As far as the numerous "research" projects that have been focused on the Bible--no amount of research can make an invalid document valid nor can it provide proof of any of the Biblical "miracles".
Further, we know that the Jews revered (what Christians call) the Old Testament so highly that they went to great lengths to ensure that new copies were identical to the documents they were copying.
No amount of care and caution in the hand-copying process could possibly prevent errors, mistranslations, and carelessness from corrupting original documents after nearly 10,000 years. Human beings can't even retain informational integrity in a game of Chinese Whispers down a chain of 6 people much less thousands upon thousands of people. This is especially true when those people have a vested interest in lending validity to themselves via the document they are translating/copying.
Not all the writers are known for certain, but we have confidence that the writings of Moses were indeed written by him, partly because Jesus (Who is God and does not make mistakes) referred to them as such.
Translation: We know the writings of Moses were indeed produced by him because a later part of the same book says they were.
Grade: FAIL!
