It's been said before that it's not entirely accurate to describe the Quran as the Islamic equivalent of the Bible. Although both are venerated as God's very word in their respective religions, Muslims' devotion to the Quran is all the more fervorous, viewing it as a very tangible incarnation of the divine on earth. Thus it wouldn't be too much of an exaggeration to say that Muslims view in the same light that Christians view the Messiah himself. This understood, one should better comprehend why, for all the exhaustive textual analysis having been performed on the Bible in both the Christian and secular traditions, Muslims continue to view any doubt or criticism as to the literal nature of the Quran as a severe blasphemy, not too different from the crucifixion of Jesus in moral sordidness.
The divine nature of the Quran is typically more important to any lay-Muslim or Islamic scholar than its explicit content; this is likely for several reasons. The manifestation of the Muhammad's revelation is not limited to the Quran itself of course, as it is braced by the far more lucid sayings of the Prophet (the hadith) as well as simple tradition. Many of the most well-known aspects of Islamic culture, thought and practice such as the 5 Pillars of Islam, the covering of women and circumcision do not know Quranic bases in themselves.
In addition, the Quran is simply a difficult to penetrate series of writings (although time and time again it touts itself as having been written in very plain prose). Muslims are known to say that the Quran cannot be understood properly in any language which is not Arabic; this is seemingly true, but it can only questionably be understood in Arabic itself. Parting from the Tanakh and Gospels, the Quran's chapters are hazily ordered by length (largest first) without any kind of reconcilable narrative. The problem of comprehensibility extends to individual chapters; much of the text could be justly described a divine stream of consciousness in which the speaker (God) rambles about in sermon form with incessant repetition and acknowledged contradiction (more on this later). An enormous amount of the vocabulary of the Quran is simply awkward or totally unknown; the reason that Quranic commentary is such an integral part of the Islamic tradition is because it has been necessary to understand and interpret certain vexing passages that even speakers of classical Arabic couldn't understand in the least.
Before it became to be considered so tactless to do so, western orientalists spoke of the Quran as a book with a palatable sense of incoherentness which confounded the pattern-seeking mind. The text borrows liberally from Bible stories and non-canonical sources, but never with attention to detail or spoken as if the audience should not already know the stories at hand. The fable of Cain and Abel is retold, but without reference to what made their sacrifices different and without their names. The story is simply told as one of two of the "sons of Adam."
The Quran of course is indeed revered as a magnificent literary document, with Muslims claiming that its beauty alone can convince the kuffar of its divine origins. That is, however, somewhat of a tautology in that it's really one of the only examples of classical Arabic composition known, with all other consecutive examples being modeled after it.
The Meccan-Medinan Distinction
The Quran serves as a compendium of holy ordinances which are typically separated into two general tendencies based on Muhammad's historical realities. Muhammad was first said to receive revelations from God in Mecca s he attempted to remove polytheist worship from the city with the apparent aid of the other village monotheists, Jews and Christians. These revelations confirm a measure of solidarity with the monotheists (known in the Quran and in popular culture as "People of the Book") and order Muslims to amicably attract new converts with God's revelation.
The temperament of the revelations changes upon Muhammad's forced exile from Mecca. When his group vacates the area and retreats to Medina, the former tolerance and cooperation with heretical monotheists whom Muhammad feels have betrayed him spoil, and with the consolidation of the Medinan state, the revelations become legal in nature and more confrontational against non-believers in the attempt to take Mecca by force.
The problem for Muslims is that divided thus, the individual surahs explicitly contradict each other in their natures. The Meccan surahs are those one will hear as presented in the essays of apologies of liberal Muslims and their advocates, claiming that Islam is indeed capable of peace and inter-faith dialog. The later Medinan surahs are typically those used as the rallying points for Salafist, Islamist and other extremist movements, as well as the common staples of the works of westerners criticizing Islam as an imperialist or dominionist ideology.
The Stories of the Quran
It's not too surprising that the Quran borrows from other holy books, but it also borrows text from non-canonical sources and the circulating folklore of the period.
A good example of this is the Quranic retelling of the Seven Sleepers, a piece of Christian folklore of Syrian origin, which had apparently been circulating recently before the first recitation of the Quran. In the story of the Seven Sleepers, seven young Christian youths flee from persecution at the hands of the Romans into a cave near Ephesus. There they pray and fast hoping for salvation while the pagans who had followed them to the cave decide to shut the cave's entrance, trapping the Christians inside. In the story, they fall asleep and God miraculously sustains them living in their slumber for three whole centuries. Eventually their cave is accidentally uncovered by a peasant, but the young men are alive and well inside but without knowledge of any time passing. They are surprised to discover that outside, the buildings of the surrounding town bear crosses on their roofs and the Roman Empire has become a Christian one. The youths realize this, tell the Christian authorities of town and die shortly after in a state of religious bliss.
The Quran makes an attempt to retell the story, but its relevancy is somewhat lost without the ability to use the backdrop of a state with a changing religion. In the Quranic version, the devout sleepers deplore idolatry in their town and take refuge in the cave (with their dog). God sustains them in sleep in the cave and they leave hundreds of years later to return to the town. The townspeople discover that they seem to be alive by miracle (when one of them uses ancient currency) and they are astonished.
The borrowings from the Tanakh and New Testament are well-known and fill the Quran, although often in fuzzy references without too much specificness. The story of Moses and the Exodus is recounted nearly a dozen times in the Quran with varying details in a way that has led many readers and listeners to think that there was never an attempt at narrative in the Quran, only an instructive usage of stories already well-known to an audience with a superficial knowledge of Jewish folklore.
It's clear that Muhammad or whoever the writers of the Quran were, not only were they acquainted with some of the Bible, but with several gospels that didn't make the final cut into the modern canon. The Quran tells the story of boy Jesus playing by the riverside, crafting birds from clay and later bringing them to life; this is a direct borrowing from the opening of the Infancy Gospel of Thomas (80-185 AD).
The birth story of Mary as well as the story of the birth of Jesus under a desert palm was mirrored only slightly before the recitation of the Quran in the Gospel of Pseudo-Matthew (600-635 AD), although it's very possible that such stories were in circulation beforehand.
The aforementioned story of Cain and Abel is suffixed by the following moral indictment:
if any one slew a person [...] it would be as if he slew the whole mankind: and if any one saved a life, it would be as if he saved the life of the whole mankind. (Surah 5.32)
which happens to be an excerpt with an origin in the Talmud of Jerusalem finalized only several hundred years before the Quran.
The Syrian Hypothesis
Both of these gospels (infancy Thomas and pseudo-Matthew) were fairly popular in the Syrian Christian tradition, bringing up another point of modern western critical analysis of the Quran. An apparently German scholar under the pseudonym “Christoph Luxenberg” gained considerable attention for building off early work on the Quran to say that the document is in reality the result of an awkward translation or commentary on some kind of Syrian Christian liturgical document. In the Syro-Aramaic Reading of the Koran, Luxenberg managed to explicate a wide variety of the ambiguous surahs by interpreting the graphemes as direct mistranslations from the Syriac script.
Hundreds of the seemingly non-sensical or confusing surahs that Arabic scholars had jumped through much rationalization to understand and explain can be easily comprehended with a reasonable knowledge of Syriac. Even the word Quran itself, typically translated into English as recitation because of its supposed dictation to Muhammad garners a new meaning; the Syriac word from which it derives is qəryānā with the connotations of the recitation being based on a written work.
The hypothesis has earned critiques and praise. Luxenberg avoids the citation of similar work and ignores, although perhaps purposefully, the work of earlier Arabic writers. This has led some to reason that he may be somewhat of an amateur on the subject. His method is also somewhat ad hoc in that any failure to interpret a passage can be remedied with an etymological twisting of the graphemes or an exchanges of the diacritical dots (which Luxenberg contends was an apparently common happening in the past). In addition scholars have noticed that Luxenberg makes several errors leading one to think that his mastery of Arabic is very modern and colloquial without too much knowledge of the known changes in the language. Regardless, Luxenberg’s hypothesis exerts extreme and unparalleled explanatory power even if he trips along the way; the idea of a non-Muhammadic origin of the Quran perhaps even in a Christianish community does seem to be gaining momentum in the western tradition.
One needn’t waste one’s breath in saying that the hypothesis has only inspired bile and anger in Muslim circles, many Muslims becoming convinced that Luxenberg is not really a German academic at all, but a despicable Syriac Christian and evangelist. His ideas must evoke a deal of hatred as they question the very consistency of classical Arabic scholarship; how can the traditional interpretations of the Quran be valid if the classic commentaries were based on a flawed understanding of the text? If anything Luxenberg’s hypothesis does explain the fact that the Quran has needed commentaries despite its professed clearness: it is not that classical Arabic scholars forgot to speak Arabic and thus left much of the work unexplained, nor is it that the Quran is simply poorly written, but it at its heart is only a partial translation of other works in a dual-tongue.