Regarding the contents of the Qur’an, Christians obviously compare it with the Gospel. In all honesty, and trying to be as objective and fair as one can be, I have to say that the Qur’an cannot be compared with Gospel. You will be convinced when you have a look at ‘Your Book and My Book’, a topical comparison of the Qur’an with the Gospel, which you can download for free from this website.
The citation of the Qur’an from memory is a different matter altogether. I remember watching dozens of young men pacing the courtyard of al-Azr University in Cairo, busy memorising the Qur’an. Sharpness of intellect, diligence and, as a bonus, a photographic memory play a part here. It would have been miraculous if this knowledge had been achieved instantly, without any learning, for example.
But let me return to your main point, the statement that the Qur’an has been preserved in its totality. While it is not possible to substantiate all my statements in a short letter like this, I will gladly do so, should you request it. It is well supported by Islamic tradition that during the lifetime of Muhammad, seven different ‘forms’ of the Qur’an existed:
“This Qur’an was revealed in seven forms, so recite what is easiest!” said Muhammad. (Al-Bukhari vol. VI, Page 482, Chapter LXI (5) Vs. 514, Mishkatul Masabih vol. 3, pp.702-704; Tafsir of at – Tabari and Commentary of al – Baidawi).
It has been suggested that this refers to different dialects. People in England, Wales, Scotland or Ireland speak very different dialects, but when they write, they use the one English language. Different dialects do not mean different texts.
We must also realise that the Uthmani version of the Qur’an is actually a revision of earlier texts. Besides the version of the Qur’an, which was collected and collated on the suggestion of Abu Bakr and Umar by Zaid b Thabith, there existed a number of other texts, compiled by men even better equipped than Zaid, men like Abdullah b. Mas’ud, Ubay b.Ka’b and Abu Moosa.