Since all of the divine books except the Quran underwent changes, how can one believe in them?

As Muslims, we believe that Moses, David and Jesus (peace be upon them) were sent the divine books called the Torah, the Psalms and the Gospel (Bible) and that those books were true and that they did not have any decrees contrary to the religion of oneness. However, those books were not protected and their originals were lost. 

We cannot say that the books that Jews and Christians have today contain nothing from the revelations sent down to prophets. However, it is a reality that superstitions and wrong beliefs were added to them. Therefore, we act cautiously towards those books. We agree that the decrees that comply with the Quran are revelations. We think that it is probable that the decrees that contradict the Quran were added afterwards. As for the decrees that neither contradict nor comply with the Quran, we do not say anything about them; we keep silent. We neither accept nor deny them because it is probable that they are revelations or not revelations.

Abu Hurayra said the following regarding the issue:

“The People of the Book read the Torah in Hebrew and translated it into Arabic for Muslims. The Messenger of Allah said the following to his companions regarding the issue:

– Neither approve nor deny the words of the People of the Book; say to them,:

“Say ye: "We believe, in Allah and the revelation given to us and to Abraham Isma`il Isaac Jacob and the Tribes and that given to Moses and Jesus and that given to (all) Prophets from their Lord we make no difference between one and another of them and we bow to Allah (in Islam)".” (Bukhari, Tafsir, Surah 2/11; I’tisam, 25)