We must as Muslims have a firm belief in the fact that Allah has sent down to His Prophets whom He has chosen, certain books. From them is the Quran, which is His speech. Also from them is the Injeel and the Taurah. We need to believe that not only the fact that they were sent down, but the fact that it was upon their people to believe in that which was sent to them through their Prophets, and live in accordance with their instructions.
This was an obligation on each of the nations before the Muslims, just as it is an obligation upon the Muslims. We must believe, as stated, that the Quran is the speech of Allah and that we must structure our lives to go in accordance with what is in the book. We must believe in every word and that which is reported as being a correct meaning of that which was revealed.
So again, we have to have a firm belief that these books that were sent down were revelation in their original forms, that were sent down from the Lord of the Worlds to each nation as a mercy and guidance.