What Islam never permitted, happened to Christianity. For the last couple of centuries Bible critics, many of them theologians, took the liberty to table and propagate their critique, which was often based on very extravagant interpretations.
We may well ask, why so many Muslims believe that the Bible was corrupted, when history, archaeology and the Qur’an deny this? The answer seems to be rather intriguing
Ibn Khazem (died AD 1064) ruled the South of Spain for some time as the vizier of the caliph. When reading the Qur’an he came across a verse that referred to Jesus speaking of Good News of an Apostle who was to come after him and whose name should be Ahmad (Surah 61:6). The meaning of this Arabic word is similar to the meaning of the name ‘Muhammad’. He also must have read about “the unlettered prophet (i.e. Muhammad) whom they find mentioned in their own (Scriptures), in the law and the Gospel” (Surah 7:157). So he began to search the Bible for these clues about Muhammad. Probably to his surprise he did not find them. What he did find, however, were a number of contradiction between the two Books, which were assumed to have come from the same divine source. We can see the problem ibn Khazem was facing.
Both, the Bible and the Qur’an, are stated to be Word of God—and they contradict each other. Ibn Khazem made the decision not to question the integrity of the Qur’an. He rather assumed that since the Gospel should agree with the Qur’an, and because Muhammad had spoken so highly of it, the existing Gospel text must have been falsified by the Jews and Christians. This assumption may display his zeal for the Qur’an, but it is not based on historical facts.