The Qur'an is the eternal miracle and the living proof of the truth of Islam. It challenged the adversaries to bring its like, even a single chapter like its, if they thought that it was not from Allah.  The enemies failed to do so; and tried to extinguish the light of Islam by waging war after war, which incidentally still continues on many fronts and in several disguises.
The most important target of the enemies of Islam was and is the Qur'an, which they have been trying to disparage in one way or the other. For example: They said that the Prophet (peace be upon him & his progeny) had learnt these profound truths from some Jews or Christians! They claimed that there were contradictory statements in the Qur'an! They pontificated that the Qur'anic ethics fell far short of the "sublime ethical teachings" of the Christianity!
None of these claims could be sustained in the eyes of independent thinkers; the Muslims rebutted all such accusations. We have only to read the talk of Amirul-mu'minin 'Ali (a.s.) with an atheist on the subject of the Qur'an, to see how the Imams of Ahlu 'l-bayt (who were the guardians of the Qur'anic truths according to the hadith of thaqalayn) defended the Qur'an and proved its pristine reality beyond any doubt. 
The purpose of this talk is to show that there was no tahrif in the Qur'an. The word tahrif may denote several meanings; however, what we are concerned with here is alteration or omission. It is our belief that there is no alteration in, addition to or omission from the Qur'an. Allah has clearly and emphatically declared that He Himself was the Protector of the Book:
Surely We have revealed the Reminder and We most surely are its Guardian. (15:9) 
In fulfillment of this promise, the Qur'an was collected in book form in the days of the Prophet (peace be upon him and his progeny) himself.
Both external evidence (from the sunnah) and internal evidence (from the Qur'an itself) have clearly established that the Prophet had left a Book in the hands of the Muslims. As for the internal evidence, there are at least fifty-two verses in which the Qur'an has been called a "Book".
As for the external evidence, the famous mutawatir hadith of Thaqalayn proves the existence of the Book during the last days of the Prophet (peace be upon him & his progeny). The hadith is narrated in scores of Shi'a and Sunni books with some minor variations in wordings, but the main parts are the same everywhere. I am quoting it from Sahih Muslim, where Zayd ibn Arqam mentions the Prophet's sermon in Ghadir Khumm:
"...And I am leaving among you two weighty things. First of them is
the Book of Allah; in it is the guidance and the light; therefore take
hold of it and adhere to it ... And (the second are) my Ahlu'l-bayt; I
remind you of Allah concerning my Ahlu 'l-bayt, I remind you of Allah concerning
my Ahlu'l-bayt, I remind you of Allah concerning my Ahlu 'l-bayt..." 
There are some other traditions (apart from the Ghadir sermon) in which the words "the Book of Allah and my traditions" have come. But the word, "Book of Allah" is present in both versions.
This sermon of Ghadir Khumm was delivered about two and a half months before the death of the Prophet and at that time there was a Book of Allah which the Prophet was leaving among his ummah and exhorting them to hold fast to it.
Then just a few days before his death, the Prophet (peace be upon him & his progeny) said:"...Come on, I will write for you a writing (because of which) you shall not go astray after me." But 'Umar said: 'Surely, the Messenger of Allah is overcome by pain, and you have got the Qur'an; the Book of Allah is sufficient for us..." 
It clearly shows that the Book of Allah was in the hands of Muslims before the death of the Prophet (peace be upon him & his progeny).
It is far-fetched to think that the word "Book" in the Qur'an, in the sermon of the Prophet, and in the statement of 'Umar was used for scattered pieces of paper, etc. The grand teacher of contemporary mujtahids, Sayyid al-Khu'i has rightly said after quoting the Qur'anic references and the hadith of Thaqalayn: "...There is a clear proof that the Qur'an was then written and compiled, because the word 'book' is not used for that which is retained in the memory, nor for scattered writings on the parchments, pieces of papers and bones, except metaphorically. But it is not right to construe any word metaphorically unless there is an evidence in its context. The word 'book' denotes existence of a collection and not of scattered scribbles, nor of things which are in the memory but not written. 
The enemies of Islam from very early days had hit upon a plan which they thought would be very effective in weakening the belief in the authenticity and originality of the Qur'an. They looked at the prevailing condition of the Muslim world and found a ready and flourishing market for forged ahadith. They exploited it to their utmost advantage and began wholesale fabrication of traditions casting aspersion on the validity of the Qur'an by imputing that there were additions, deletions, alterations and even some mistakes in this Qur'an which is in our hands.
This idea was and is diametrically opposed to what the Muslims, the Shi'a and the Sunni alike, believe about the Qur'an. Shaykh Abu Ja far as-Saduq (d. 381) writes in his Kitabu 'l-I'tiqadat:
"It is our belief that the Qur'an which Allah revealed to His Prophet Muhammad is (the same as) the one between the two boards (daffatayn). And it is that which is in the hands of the people, and is not greater in extent than that."
Then he says:
"And he who asserts that we say that it is greater in extent than this (the present text) is a liar." 
Similar declarations have been made by almost all great Shi'a mujtahids; for example, Shaykh al-Mufid (338-413), Awa'ilu l-Maqalat, p. 95; Sharif al-Murtada (355-436), Bahru 'l-Fawa'id (Tehran, 1314) p. 69; Shaykh at-Tusi (385-408), Tafsir at-Tibyan, vol. 1 (Najaf, 1376) p. 3; Shaykh at-Tabrasi (d. 548), Majma'u 'l-Bayan, vol. 1 (Lebanon) p. 15.
This belief has continued un-interrupted upto the present days. One may just mention the names of Sayyid Muhsin al-Amin al-'Amili (1284-1371); Sayyid Sharafu'd-Din al-Musawi (1290-1377); Shaykh Muhammad Husayn Kashifu 'l-Ghita' (1295-1373); Sayyid Muhsin al-Hakim (1306- 1390); 'Allamah at-Tabataba'i (1321- 1402) Sayyid Muhammad Hadi al-Milani (1313-1390); Sayyid Ruhullah al-Khumayni (1321 - 1409); Sayyid Abu 'l-Qasim al-Khu'i (1317-1413) and Sayyid Muhammad Rida al-Gulpaygani (1316-1414).
I would like to give short hints on the following six aspects of this topic, to show how the enemies of islam got the chance to create the fitnah of tahrif and to what extent they succeeded in their endeavours.
1. The appearance of forged ahadith in the Muslim world.
2. Confessions of some of those who indulged in this practice.
3. Examples of forged ahadith showing that hundreds, if not thousands, of verses were deleted from the Qur'an.
4. Sunni attitude and their belief. The theory of abrogation of recitation and its implication.
5. Shi'a attitude towards such traditions.
6. American Islam resurrected this buried topic, ostensibly to ridicule Shi'ism and hinder Iranian efforts for establishing Islamic unity world-wide. But what was its real motive?
Obviously the time does not allow to go into details. Therefore, I propose only to mention briefly these topics in this talk.
See the Qur'an 17:88; 11:11; 10:38.
At-Tabrasi. Al-Ihtijaj vol I (Najaf: Daru n-Numarn 1966) pp.358-384.
Note the triple emphasis of the nominal clause (al-jumlatu 'l-ismiyya),
inna and lam at-tak'id.
Sahih Muslim, vol 4 (Beirut 2nd edition. 1972) p 1873. This hadith
has been narrated by more than 20 companions See also, Musnad Ahmad
bin Hanbal vol. 3 pp 14. 17. 26.59: vol 4. pp 366. 371: vol. 5. pp
182. 189 Sahih at-Tirmidhi vol. 3 (chap. "Manaqib Ahli l-bayt"
) p. 200-102
Sahih Muslim, vol. 3 (Beirut. 1st edition. 19O5/1375)p.1295. Bukhari
has given this tradition in four places Vide ,Sahih Bukhari (Cairo:
1958)vol. 1 ("Kitabu 'l-ilm: bab kitabatu 'l- ilm ) p. 39: vol. 6 ("Bab
kitabu 'n-Nabi ila Kasra wa Qaysar) pp 11-12: vol.7 ( Kitabu 't-tibb: bab
qawli 'l-marid Qumu 'anni') pp. 155-156: vol. 9 ("Kitabu l-itisam bi l-kitab
wa s-sunnah: bab karahiyyati 'l-khilaf') p. 137.
Al-Khui, al-Bayan Tafsiri 'l-Qur'an (Kuwait. 1399/1979) p. 271.
As-Saduq, Kitabu 'I-Itiqadat. (Tehran: 1370 AH) p. 63. See
also its translation The Shi'ite Creed, tr. A.A.A. Fyzee (Calcutta: 1942)