The Book of God

We Muslims believe that Quran is the only book which has not tempered so do other Abrahamic religion followers for their books. Christians and Jews believe in their books and take rest of the scriptures as 'fake' and this is the biggest dispute amongst Muslims. Christians and Jews.

My query is Why Allah did not preserve the law of Moses and the gospel of Jesus, He has preserved the message of Quran (as we claim) but If Allah could preserve the message of the Quran, He could have preserved His Word before then too.

Why Allah only promised to preserve his One book and why left other books to be corrupted? Were not those the Holy book and that too by HIM ? Don't you think calling other scriptures tempered is actually doubting the reputation of messengers whose efforts went in vain at the end?