Much of the content in the Qur'an makes reference back to parts of the Christian Bible. Well-known Biblical characters such as Abraham, Noah, and Jesus are mentioned. Detractors of the Qur'an and Muhammad have claimed that Muhammad was merely taking older religious documents and stories and embellishing them. Muslims maintain that this could not be the case, on scientific grounds. Modern science has identified many factual errors and internal inconsistencies in the Bible. However, the Qur'an has none, according to Muslims; and what's more, in cases where Biblical stories reappear in very similar form in the Qur'an, the stories are always changed in such a way that the scientific inaccuracies and inconsistencies are gone. This is considered proof of the Qur'an's divine origin.
The Qur'an today is believed to be identical to the version from the time of Muhammad. Muhammad only delivered the Qur'an in spoken form during his lifetime; the word "Qur'an" in fact means "the recitation." The faithful were require to memorize them perfectly, down to the last syllable, and recite them frequently, to make sure they had the text perfectly. Shortly after his death, his disciples began recording all the Suras down in written form. Thus, two different mechanisms were in place -- oral and written -- to help ensure that no corruption of the text took place over time. There is almost no dispute amongst modern Qur'anic scholars that the text today is as it was when it was first written down.