There are hundreds of religions flourishing around the world: Christianity, Islam, Buddhism, Sikhism, Hinduism, Judaism, Bahaism, Babism, Zoroastrianism, Mormonism, Jehovas Witnesses, Jainism, Confucianism etc. And each of these religions claim that their scripture is preserved from the day it was revealed (written) until our time. A religious belief is as authentic as the authenticity of the scripture it follows. And for any scripture to be labeled as authentically preserved it should follow some concrete and rational criteria.
Imagine this scenario: A professor gives a three hour lecture to his students. Imagine still that none of the students memorized this speech of the professor or wrote it down. Now forty years after that speech, if these same students decided to replicate professor's complete speech word for word, would they be able to do it? Obviously not. Because the only two modes of preservation historically is through writing and memory. Therefore, for any claimants to proclaim that their scripture is preserved in purity, they have to provide concrete evidence that the Scripture was written in its entirety AND memorized in its entirety from the time it was revealed to our time, in a continuous and unbroken chain. If the memorization part doesn't exist parallel to the written part to act as a check and balance for it, then there is a genuine possibility that the written scripture may loose its purity through unintentional and intentional interpolations due to scribal errors, corruption by the enemies, pages getting decomposed etc, and these errors would be concurrently incorporated into subsequent texts, ultimately loosing its purity through ages.
Now, of all the religions mentioned above, does any one of them possess their scriptures in its entirety BOTH in writing AND in memory from the day of its revelation until our time. None of them fit this required criteria, except one: This unique scripture is the Qur'an - revelation bestowed to Prophet Muhammad (peace be upon him) 1,418 years ago, as a guidance for all of humankind.
Transmission of the Qur'an: Oral & Written
Lets analyze the claim of the preservation of the Quran…
'In the ancient times, when writing was scarcely used, memory and oral transmission was exercised and strengthened to a degree now almost unknown' relates Michael Zwettler.
Prophet Muhammad (S): The First Memorizer
It was in this 'oral' society that Prophet Muhammad (S) was born in Mecca in the year 570 C.E. At the age of 40, he started receiving divine Revelations from the One God, Allah, through Archangel Gabriel. This process of divine revelations continued for about 22.5 years just before he passed away. Prophet Muhammad (S) miraculously memorized each revelation and used to proclaim it to his Companions. Angel Gabriel used to refresh the Quranic memory of the Prophet each year. 'The Prophet (S) was the most generous person, and he used to become more so (generous) particularly in the month of Ramadan because Gabriel used to meet him every night of the month of Ramadan till it elapsed. Allah's Messenger (S) use to recite the Qur'an for him. When Gabriel met him, he use to become more generous than the fast wind in doing good'.  'Gabriel used to repeat the recitation of the Qur'an with the Prophet (S) once a year, but he repeated it twice with him in the year he (Prophet) died'.  The Prophet himself use to stay up a greater part of the night in prayers and use to recite Quran from memory.
Prophet's Companions: The First Generation Memorizers
Prophet Muhammad (S) encouraged his companions to learn and teach the Quran: 'The most superior among you (Muslims) are those who learn the Qur'an and teach it'.  'Some of the companions who memorized the Quran were: 'Abu Bakr, Umar, Uthman, Ali, Ibn Masud, Abu Huraira, Abdullah bin Abbas, Abdullah bin Amr bin al-As, Aisha, Hafsa, and Umm Salama'.  'Abu Bakr, the first male Muslim to convert to Islam used to recite the Quran publicly in front of his house in Makka'.  The Prophet also listened to the recitation of the Qur'an by the Companions: 'Allah Apostle said to me (Abdullah bin Mas'ud): "Recite (of the Quran) to me". I said: "Shall I recite it to you although it had been revealed to you?!" He Said: "I like to hear (the Quran) from others". So I recited Sura-an-Nisa' till I reached: "How (will it be) then when We bring from each nation a witness and We bring you (O Muhammad) as a witness against these people?"' (4:41) 'Then he said: "Stop!" Behold, his eyes were shedding tears then'.  Many Quranic memorizers (Qurra) were present during the lifetime of the Prophet and afterwards through out the then Muslim world.'At the battle of Yamama, many memorizers of the Quran were martyred. 'Narrated Zaid bin Thabit al Ansari, who was one of those who use to write the Divine Revelations: Abu Bakr sent me after the (heavy) casualties among the warriors (of the battle) of Yamama (where a great number of Qurra were killed). Umar was present with Abu Bakr who said: "Umar has come to me and said, the people have suffered heavy casualties on the day of (the battle of) Yamama, and I am afraid that there will be some casualties among the Qurra (those who memorized the entire Quran) at other place…"'  'Over the centuries of the Islamic Era, there have arisen throughout the various regions of the Islamic world literally thousands of schools devoted specially to the teaching of the Quran to children for the purpose of memorization. These are called, in Arabic, katatib (singular: Kuttab). It is said that the Caliph 'Umar (634-44) first ordered the construction of these schools in the age of the great expansion'. 
Second Generation Memorizers:
"…Quranic schools were set up everywhere. As an example to illustrate this I may refer to a great Muslim scholar, of the second Muslim generation, Ibn 'Amir, who was the judge of Damascus under the Caliph Umar Ibn 'Abd Al-Aziz. It is reported that in his school for teaching the Quran there were 400 disciples to teach in his absence". 
Memorizers in Subsequent Generations:
The Number of Katatib and similar schools in Cairo (Egypt) alone at one time exceeded two thousand.  Currently both in the Muslim and non-Muslim countries thousands of schools with each instructing tens of hundreds of students the art of memorizing the entire Quran. In the city of Chicago itself, there are close to 40+ Mosques, with many of them holding class for children instructing them the art of Quranic memorization.
Further Points of Consideration:
Muslims recite Quran from their memory in all of their five daily prayers.
Once a year, during the month of Fasting (Ramadan), Muslims listen to the complete recitation of the Quran by a Hafiz (memorizer of the entire Quran)
It's a tradition among Muslims that before any speech or presentation, marriages, sermons, Quran is recited.
Quran is the only book, religious or secular, on the face of this planet that has been completely memorized by millions. These memorizers range from ages 6 and up, both Arabic and non-Arabic speakers, blacks, whites, Orientals, poor and wealthy.Thus the process of memorization was continuous , from Prophet Muhammad's (S) time to ours with an unbroken chain."The method of transmitting the Quran from one generation to the next by having he young memorize the oral recitation of their elders had mitigated somewhat from the beginning the worst perils of relying solely on written records…" relates John Burton.  "This phenomenon of Quranic recital means that the text has traversed the centuries in an unbroken living sequence of devotion. It cannot, therefore, be handled as an antiquarian thing, nor as a historical document out of a distant past. The fact of hifz (Quranic Memorization) has made the Qur'an a present possession through all the lapse of Muslim time and given it a human currency in every generation never allowing its relegation to a bare authority for reference alone" reflects Kenneth Cragg. 
2. Written Text of the Quran
Prophet Muhammad (S) was very vigilant in preserving the Quran in the written form from the very beginning up until the last revelation. The Prophet himself was unlettered, did not knew how to read and write, therefore he called upon his numerous scribes to write the revelation for him. Complete Quran thus existed in written form in the lifetime of the Prophet.Whenever a new revelation use to come to him, the Prophet would immediately call one of his scribes to write it down. 'Some people visited Zaid Ibn Thabit (one of the scribes of the Prophet) and asked him to tell them some stories about Allah's Messenger. He replied: "I was his (Prophet's) neighbor, and when the inspiration descended on him he sent for me and I went to him and wrote it down for him…" Narrated by al-Bara': There was revealed 'Not equal are those believers who sit (home) and those who strive and fight in the cause of Allah' (4:95). The Prophet said: 'Call Zaid for me and let him bring the board, the ink pot and scapula bone.' Then he (Prophet) said: 'Write: Not equal are those believers…'  Zaid is reported to have said: 'We use to compile the Qur'an from small scraps in the presence of the Apostle'. 'The Prophet, while in Madinah, had about 48 scribes who use to write for him'. Abdullah Ibn 'Umar relates:… 'The Messenger of Allah (S) said: "Do not take the Qur'an on a journey with you, for I am afraid lest it should fall into the hands of the enemy"'  During the Prophet's last pilgrimage, he gave a sermon in which he said: 'I have left with you something which if you will hold fast to it you will never fall into error - a plain indication, the Book of God (Quran) and the practice of his Prophet…'  'Besides the official manuscripts of the Quran kept with the Prophet, many of his companions use to possess their own written copies of the revelation'. 'A list of Companions of whom it is related that they had their own written collections included the following: Ibn Mas'ud, Ubay bin Ka'b, Ali, Ibn Abbas, Abu Musa, Hafsa, Anas bin Malik, Umar, Zaid bin Thabit, Ibn Al-Zubair, Abdullah ibn Amr, Aisha, Salim, Umm Salama, Ubaid bin Umar'. 'The best known among these (Prophet's Scribes) are: Ibn Masud, Ubay bin Kab and Zaid bin Thabit'. 'Aisha and Hafsa, the wives of the Prophet had their own scripts written after the Prophet had died'. 
The complete Quran was written down in front of the Prophet by several of his scribes and the companions possess their own copies of the Quran in the Prophet's lifetime. However the written material of the Quran in the Prophet's possession were not bounded between the two covers in the form of a book, because the period of revelation of the Qur'an continued up until just a few days before the Prophet's death. The task of collecting the Qur'an as a book was therefore undertaken by Abu Bakr, the first successor to the Prophet.
Written Qur'an in First Generation
At the battle of Yamama (633 CE), six months after the death of the Prophet, a number of Muslims, who had memorized the Quran were killed. Hence it was feared that unless a written official copy of the Quran were prepared, a large part of revelation might be lost. Narrated Zaid bin Thabit al-Ansari, one of the scribes of the Revelation: Abu Bakr sent for me after the casualties among the warriors (of the battle) of Yamama (where a great number of Qurra (memorizers of the Quran, were killed). Umar was present with Abu Bakr who said: "Umar has come to me and said, the people have suffered heavy casualties on the day of (the battle) of Yamama, and I am afraid that there will be some casualties among the Qurra at other places, whereby a large part of the Quran may be lost, unless you collect it (in one manuscript, or book)…so Abu Bakr said to me (Zaid bin Thabit): You are a wise young man and we do not suspect you (of telling lies or of forgetfulness) and you used to write the Divine Inspiration for Allah's Apostle. Therefore, look for the Qur'an and collect it (in one manuscript)'…So I started locating the Quranic material and collecting it from parchments, scapula, leafstalks of date palms and from the memories of men (who know it by heart)…" Now, a committee was formed to under take the task of collecting the written Quranic material in the form of a book. The committee was headed by Zaid bin Thabit, the original scribe of the Prophet, who was also a memorizer of the complete Quran.'…Zaid bin Thabit had committed the entire Quran to memory…' 
The compilers in this committee, in examining written material submitted to them, insisted on very stringent criteria as a safeguard against any errors.
1. The material must have been originally written down in the presence of the Prophet; nothing written down later on the basis of memory alone was to be accepted. 
2. The material must be confirmed by two witnesses, that is to say, by two trustworthy persons testifying that they themselves had heard the Prophet recite the passage in question. 
'The manuscript on which the Qur'an was collected, remained with Abu Bakr till Allah took him unto Him, and then with Umar (the second successor), till Allah took him unto Him, and finally it remained with Hafsa, 'Umar's daughter (and wife of the Prophet)'. This copy of the Quran, prepared by the committee of competent companions of the Prophet (which included Memorizers of the Quran) was unanimous approved by the whole Muslim world. If they committee would have made a error even of a single alphabet in transcribing the Quran, the Qurra (memorizers of the Quran) which totaled in the tens of hundreds would have caught it right away and correct it. This is exactly where the neat check and balance system of preservation of the Quran comes into play, but which is lacking for any other scripture besides the Quran.
Official written copy by Uthman
The Quran was originally revealed in Quraishi dialect of Arabic. But to facilitate the people who speak other dialects, in their understanding and comprehension, Allah revealed the Quran finally in seven dialects of Arabic. During the period of Caliph Uthman (second successor to the Prophet) differences in reading the Quran among the various tribes became obvious, due to the various dialectical recitations. Dispute was arising, with each tribe calling its recitation as the correct one. This alarmed Uthman, who made a official copy in the Quraishi dialect, the dialect in which the Quran was revealed to the Prophet and was memorized by his companions. Thus this compilation by Uthman's Committee is not a different version of the Quran (like the Biblical versions) but the same original revelation given to the Prophet by One God, Allah.Narrated Anas bin Malik: Hudhaifa bin Al-Yaman came to Uthman at the time when the people of Sham (Syria) and the people of Iraq were waging war to conquer Armenia and Azherbijan. Hudhaifa was afraid of their differences in the recitation of the Quran, so he said to Uthman, 'O chief of the Believers! Save this nation before they differ about the Book (Quran) as Jews and Christians did before'. So Uthman sent a message to Hafsa saying, 'Send us the manuscripts of the Quran so that we may compile the Quranic materials in perfect copies and return the manuscripts to you'. Hafsa sent it to Uthman. 'Uthman then ordered Zaid bin Thabit, 'Abdullah bin Az-Zubair, Said bin Al-As and Abdur Rahman bin Harith bin Hisham to rewrite the manuscripts in perfect copies. Uthman said to the three Quraishi men, 'In case you disagree with Zaid bin Thabit on any point in the Quran, then write it in their (Quraishi) tongue'. They did so, and when they had written many copies, Uthman sent to every Muslim province one copy of what they had copied and ordered that all the other Quranic materials whether written in fragmentary manuscripts or whole copies, be burnt…" 
Again a very stringent criteria was set up by this Committee to prevent any alteration of the Revelation.
1. The earlier recension (Original copy prepared by Abu Bakr) was to serve as the principal basis of the new one. 
2. Any doubt that might be raised as to the phrasing of a particular passage in the written text was to be dispelled by summoning persons known to have learned the passage in question from the Prophet. 
3. Uthman himself was to supervise the work of the Council. 
When the final recension was completed, Uthman sent a copy of it to each of the major cities of Makka, Damascus, Kufa, Basra and Madina.The action of Uthman to burn the other copies besides the final recension, though obviously drastic, was for the betterment and harmony of the whole community and was unanimously approved by the Companions of the Prophet.Zaid ibn Thabit is reported to have said: "I saw the Companions of Muhammad (going about) saying, 'By God, Uthman has done well! By God, Uthman has done well!" Another esteemed Companion Musab ibn Sad ibn Abi Waqqas said: "I saw the people assemble in large number at Uthman's burning of the prescribed copies (of the Quran), and they were all pleased with his action; not a one spoke out against him". Ali ibn Abu Talib, the cousin of the Prophet and the fourth successor to the Prophet commented: "If I were in command in place of Uthman, I would have done the same". Of the copies made by Uthman, two still exist to our day. One is in the city of Tashkent, (Uzbekistan) and the second one is in Istanbul (Turkey). Below is a brief account of both these copies:
1. The copy which Uthman sent to Madina was reportedly removed by the Turkish authorities to Istanbul, from where it came to Berlin during World War I. The Treaty of Versailles, which concluded World War I, contains the following clause:'Article 246: Within six months from the coming into force of the present Treaty, Germany will restore to His Majesty, King of Hedjaz, the original Koran of Caliph Othman, which was removed from Madina by the Turkish authorities and is stated to have been presented to the ex-Emperor William II". 'This manuscript then reached Istanbul, but not Madina (Where it now resides)'. 2. The second copy in existence is kept in Tashkent, Uzbekistan. 'It may be the Imam (master) manuscript or one of the other copies made at the time of Uthman'. It Came to Samarkand in 890 Hijra (1485) and remained there till 1868. Then it was taken to St.Petersburg by the Russians in 1869. It remained there till 1917. A Russian orientalist gave a detailed description of it, saying that many pages were damaged and some were missing. A facsimile, some 50 copies, of this mushaf (copy) was produced by S.Pisareff in 1905. A copy was sent to the Ottoman Sultan 'Abdul Hamid, to the Shah of Iran, to the Amir of Bukhara, to Afghanistan, to Fas and some important Muslim personalities. One copy is now in the Columbia University Library (U.S.A.). 'The Manuscript was afterwards returned to its former place and reached Tashkent in 1924, where it has remained since'. 
'Two of the copies of the Qur'an which were originally prepared in the time of Caliph Uthman, are still available to us today and their text and arrangement can be compared, by anyone who cares to do, with any other copy of the Quran, be it in print or handwritten, from any place or period of time. They will be found identical'. It can now be proclaimed, through the evidences provided above, with full conviction and certainty that the Prophet memorized the entire Quran, had it written down in front of him through his scribes, many of his companions memorized the entire revelation and in turn possess their own private copies for recitation and contemplation. This process of dual preservation of the Quran in written and in the memory was carried in each subsequent generation till our time, without any deletion, interpolation or corruption of this Divine Book.Sir Williams Muir states, " There is otherwise every security, internal and external, that we possess the text which Muhammad himself gave forth and used". Sir William Muir continues, "There is probably no other book in the world which has remained twelve centuries (now fourteen) with so pure a text".  This divine protection provided to the Quran, the Last Reveled Guide to Humanity, is proclaimed by One God in the Quran: 'We* (Allah) have, without doubt, send down the Message; and We will assuredly Guard it (from corruption)' (Quran - Chapter 15, Verse 9).*('We' is the plural of Majesty, and not the Christian plural of trinity)Compare this divine and historical preservation of the Quran with any literature, be it religious or secular and it becomes evident that none possess similar miraculous protection. And as states earlier, a belief is as authentic as the authenticity of its scripture. And if any scripture is not preserved, how can we be certain that the belief arising out of this scripture is divine or man made, and if we are not sure about the belief itself, then our salvation in the hereafter would be jeopardized.
Thus this above evidence for the protection of the Quran from any corruption is a strong hint about its divine origin. We request all open hearted persons to read, understand and live the Quran, the 'Manual for Mankind'.
1. Michael Zwettler, The Oral Tradition of Classical Arabic Poetry, p.14. Ohio State Press: 1978.
2. Transmitted by Ibn Abbas, collected in Sahih Al-Bukhari, 6.519, translated by Dr. Muhammad Muhsin Khan.
3. Transmitted by Abu Hurayrah, collected in Sahih Al-Bukhari, 6.520, translated by Dr. Muhammad Muhsin Khan.
4. Transmitted by Uthman bin Affan, collected in Sahih Bukhari, 6.546, translated by Dr. Muhammad Muhsin Khan.
5. Jalal al-Din Suyuti, 'Al-Itqan fi-ulum al-Quran, Vol. I, p. 124.
6. Ibn Hisham, Sira al-nabi, Cairo, n.d., Vol.I, p. 206.
7. Al-Bukhari, 6.106.
8. Al-Bukhari, 6.201.
9. Labib as-Said, The Recited Koran, tr. Bernard Weiss, M.A.Rauf, and Morroe Berger, The Darwon Press, Princton, New Jersey, 1975, pg. 58.
10. Ibn al Jazari, Kitab al-Nash fi al-Qir'at al-Ashr, Cairo, al-Halabi, n.d._ vol. 2, p. 254; also Ahmad Makki al-Ansari, al-Difa' 'An al-Qur'an. Cairo, Dar al-Ma'arif, 1973 C.E., part I, p. 120.
11. Labib as-Said, The Recited Koran, tr. Bernard Weiss, M.A.Rauf, and Morroe Berger, The Darwon Press, Princeton, New Jersey, 1975, pg. 59.
12. John Burton, An Introduction to the Hadith, Edinburgh University Press: 1994, p. 27.
13. Kenneth Cragg, The Mind of the Qur'an, George Allah & Unwin: 1973, p.26.
14. Tirmidhi, Mishkat al-Masabih, No. 5823.
15. Al-Bukhari, 6.512.
16. Suyuti, Itqan, I, p. 99.
17. M. M. Azami, Kuttab al-Nabi, Beirut, 1974.
18. Muslim, III, No. 4606; also 4607, 4608; Bukhari, 4.233.
19. Ibn Hisham, Sira al-nabi, p. 651.
20. Suyuti, Itqan, I, p. 62.
21. Ibn Abi Dawud, Masahif, p. 14.
22. Bayard Dodge, The fihrist of al-Nadim: A Tenth Century Survey of Muslim Culture, New York, 1970, pp. 53-63.
23. Imam Malik, Muwatta, tr. M. Rahimuddin, Lahore, 1980, no.307, 308.
24. Bukhari, 6.201.
25. Labib as-Said, The Recited Koran, tr. Bernard Weiss, et al., 1975, p. 21.
26. Ibn Hajar, Fath, Vol. IX, p. 10.
27. ibid., p. 11.
28. Bukhari, 6.201.
29. Bukhari, 6.510.
30. Ibn Hajar, Bath, IX, p. 15.
31. Suyuti, Itqan, Vol.I, p. 59.
32. ibid., p. 59.
33. Naysaburi, al-Nizam al-Din al-Hasan ibn Muhammad, Ghara'ib al-Quran wa-ragha'ib al-furqan, 4 vols., to date. Cairo, 1962.
34. Ibn Abi Dawud, p. 12.
35. Zarkashi, al-Badr al-Din, Al-Burhan fi-ulum al-Quran, Cairo, 1957, vol. I, p. 240.
36. Fred L. Israel, Major Peace Treaties of Modern History, New York, Chelsea House Pub., Vol. II, p. 1418.
37. Makhdum, op.cit., 1938, p. 19.
38. Ahmad Von Denffer, Ulum Al-Qur'an, revised ed., Islamic Foundation, 1994, p. 63.
39. The Muslim World, vol. 30 (1940), pp. 357-8.
40. Ahmad von Denffer, Ulum Al-Quran, revised ed., Islamic Foundation, 1994, p. 63.
41. ibid., p. 64.
42. Sir Williams Muir, Life of Mohamet, vol.1, Introduction.
26 February 1998.
-----------------------------[Currently, he is the co-chairman of the Da'wa Committee and Board of Director at the Muslim Community Center, Illinois. A student of Ahmed Deedat and now works for the 1-800-662-islam hotline of ICNA. His main interest is in comparative religion.]