The Koran is a scripture sacred to Islam.
The Koran is claimed by Muslims to have been revealed to Muhammad over a number of years in the seventh century, purportedly through the angel Gabriel. The scripture is organized (roughly) according to the length of the chapters. There are 114 suras (or chapters), with many of the later and more warlike ones appearing near the beginning. The Prophet's first revelations were received in Mecca. He was exiled from Mecca after commanding the people to repent of polytheism and join the One True God, (in Arabic, his name is Allah). The Prophet Muhammad then traveled to Medina and converted the entire city. He led them to a victory over the polytheists at Mecca.
The verses are of two kinds: Meccan and Medinan. Meccan verses are characterized by praise of a monotheistic God and condemnation of polytheism. Medinan verses reflect divine laws and commandments that would help Muhammad in his new role as a leader of men; that is to say, the verses are concerned more with the social laws and obligations of Muslims.
The Koran has influenced a great number of people over the centuries, especially in the Middle East. (A link to the text of the Koran and translations can be found below.)
Several commentaries on the Koran exist. These are labelled as "tafsir" (the word for "commentary.") Allah himself is regarded as the greatest Tafsir. Muhammad is the second-best (his tafsir is called "hadith," the Words of the Prophet). The Companions of the Prophet also produced their own tafsirs. It is important to note that "tafsir" is not scripture; it is a companion to understanding scripture.
It is also important to note that Muhammad is not worshiped as a God. The Muslims are monotheistic. The Koran states that God has no equals, nor was he begotten by a greater being, nor has he begotten any other being. Muslims may occasionally view Jesus and the Holy Spirit as infringing on God's divinity. The Christian doctrine of the Holy Trinity is regarded as tantamount to polytheism by Muslims.
The Koran was revealed to Muhammad in what the Angel said was "clear Arabic." This has been interpreted to mean that Arabic is the most beautiful language of all. The Koran also contains a challenge: if you do not believe that the Koran is the word of God, try to make something just as beautiful and good as it! This is the doctrine of inimitability (no copy can be made), or "i'jaz." These two doctrines, along with the inherent difficulty of translation, severely limit the value of translations of the Koran. In other words, the only Koran is the Arabic Koran. English versions of the Koran are regarded as commentaries.
Two of the most famous and widely-read English translations of the Koran are those by A. Yusuf Ali and M. Marmaduke Pickthall  (and see links below).
Every major English-language dictionary gives "Koran" as its preferred spelling, including American Heritage, Merriam-Webster, Webster's New World College Dictionary, and Oxford. The Chicago Manual of Style, the most widely followed style guide, recommends "Koran." The Associated Press, which sets a style benchmark for much of the media, has used "Quran" since 2000. This is despite the fact that both dictionaries recommended in the AP Stylebook give the K spelling. "Quran" and "Qu'ran," another common varient, are both simplified forms of the romanized Arabic (al-Qur'ān).
Scholars and theologians have identified six main sources for the contents of the Koran. The Catholic Encyclopedia states: "The sources of the Koran be reduced to -
- The Old Testament (canonical and apocryphal) and the hybrid Judaism of the late rabbinical schools. During Mohammed's time the Jews were numerous in many parts of Arabia, especially around Medina. Familiarity with them is undoubtedly responsible for many Old Testament stories alluded to in Koran. Later Judaism and Rabbinism are equally well represented (Geiger, "Was hat Mohammed aus dem Judenthum aufgenommen?", Wiesbaden, 1833; tr. "Judaism and Islam", Madras, 1898).
- The New Testament (canonical and apocryphal) and various heretical doctrines. On his journeys between Syria, Hijaz, and Yemen, Mohammed had every opportunity to come in close touch with Yemenite, Abyssinian, Ghassanite, and Syrian Christians, especially heretics. Hence, while the influence of orthodox Christianity upon the Koran has been slight, apocryphal and heretical Christian legends, on the other hand, are one of the original sources of Koranic faith. (See Muir, op. cit. infra, 66-239; Tisdall, "The Original Sources of the Qur'an", London, 1905, 55-211.)
- Sabaism, a combination of Judaism, Manicheism, and old disfigured Babylonian heathenism.
- Zoroastrianism. On account of Persia's political influence in the north-eastern part of Arabia, it is natural to find Zoroastrian elements in the Koran.
- Hanifism, the adherents of which, called Hanifs, must have been considerable in number and influence, as it is known from contemporary Arabian sources that twelve of Mohammed's followers were members of this sect.
- Native ancient and contemporary Arabian heathen beliefs and practices. Wellhausen has collected in his "Reste des arabischen Heidentums" (Berlin, 1897) all that is known of pre-Islamic Arabian heathen belief, traditions, customs, and superstitions, many of which are either alluded to or accepted and incorporated in the Koran. From the various sects and creeds, and Abul-Fida, the well-known historian and geographer of the twelfth century, it is clear that religious beliefs and practices of the Arabs of Mohammed's day form one of the many sources of Islam. From this heathen source Islam derived the practices of polygamy and slavery, which Mohammed sanctioned by adopting them."
Dependence upon the Bible
The Koran manifests that it depends upon the Bible for much of its content, and that the Bible was preserved by God. "The Koran its hearers as though they are already familiar with the stories of the great biblical figures, commenting on them without retelling the narrative...It has often been noted that these figures are presented in the Koran with little of the detail and few of the distinguishing characteristics found in the biblical narratives.
The Bible versus the Koran
At less than 80,000 words, (figures vary: 77,701 in Arabic by one source, 77,439 by ‘Ata bin Yasar) the Koran is much smaller than the Bible, (602,585 words in the Old Testament; 180,552 in the New = 783,037, with the KJV being counted at 788,258 words total.) The Koran is also far more restrictive in in its scope of communication, lacking the manner of extensive historical narratives of the Bible, and the context it provides for its commands, as well as genealogical records which helps provide historical chronology. In addition, absent from the Koran are extended doctrinal discourses on salvation, such as are especially seen in the New Testament. This results in difficultly formulating extensive systematic theology out of the Koran by itself, and in its allusions to Biblical characters and events evidences that it requires knowledge of the Bible, which as the prior revelation, it must agree with. Muhammed himself is seen to uphold the Scriptures that existed then as divinely inspired, both the Torah, (Sura 2:87) and the Psalms, (4:163) and the Gospels, (Suras 3:3; 5:46) such as in stating,
If thou wert in doubt as to what We have revealed unto thee, then ask those who have been reading the Book from before thee: the Truth hath indeed come to thee from thy Lord: so be in no wise of those in doubt. 10:94 (Y. Ali)
Say: "O People of the Book! ye have no ground to stand upon unless ye stand fast by the Law, the Gospel, and all the revelation that has come to you from your Lord, (Sura 5:68; Y. Ali) and
And dispute ye not with the People of the Book, except with means better (than mere disputation), unless it be with those of them who inflict wrong (and injury): but say, "We believe in the revelation which has come down to us and in that which came down to you; Our Allah and your Allah is one; and it is to Him we bow (in Islam). (Sura 29:46; Y. Ali)
Sura 6:34 also states that "there is none that can alter the words (and decrees) of Allah."
However, the Koran critically contradicts the Bible, especially as concerns the person and work of Jesus Christ, as it flatly denies His death and resurrection, as well as His Divine nature, both of which which are held by Christians to be abundantly testified to.
Islamic apologists recognize these contradictions, and therefore charge that the Bible was tampered with, though the amount of alterations required to explain the Koranic deviations from Biblical text would require a radical amount of rewriting, especially of the latter.
Christian apologists respond to these charges by evidencing that an abundance of manuscripts which predate the Koran yet exist, such as the Codex Sinaiticus (c. 350 AD), with Aland numbering a total of 230 extant New Testament manuscript portions which pre-date 600 AD (192 Greek New Testament manuscripts, five Greek lectionaries containing scripture, and thirty-three translations of the Greek New Testament). The doctrinal conflation of these with later manuscripts manifest that the alleged rewriting did not take place.
In addition, it is postulated that if Muhammed could not read, as Islam states, then he would have relied upon the word of others (travelers, etc.) who were overall likely to be significantly Biblically illiterate. This would explain how Muhammed could reprove such things as the Christian Trinity consisting of God, Jesus and Mary, (Sura 5:116-117) in addition to speaking numerous contradictions of the Bible.
In response, Muslim apologists seek to make a case against a reliable Bible based upon the lack of a universal canon among all Christian churches, or early incomplete manuscripts, or additions to them (the Epistle of Barnabas, and portions of The Shepherd of Hermas to Codex Sinaiticus), as well as evidence of a relatively small degree of actual (beyond spelling, etc.) textual disagreement among some of the thousands of existing manuscripts. However, none of these aspects provides a solution to the Islamic problem, as none of the manuscripts substantiates the Koranic deviations, nor does the small percentage (contrary to typical Muslim exaggeration) of actual variants among manuscripts support the manner of massive changes which the Islamic solution requires. In addition, the Koran has its own problematic texts, even though it is a far smaller (in size and scope) book, and one that was written after durable writing materials came into use, and then underwent a purification process in which variant manuscripts were burned. Thus it is seen that the manner of corruption of biblical text which Islam charges is that of the Koran, not the Bible which it is largely based upon.
While Muslims also typically claim that the text of the Koran is identical to that received by Muhammad. Christian researchers contend that there is overwhelming evidence that it is not. 
As no extant Biblical manuscript, from before the time of Muhammed or after him, agrees with the Koranic contradictions, many Muslim apologists look to the Gospel of Barnabas for support. However, this radically different account is judged by both Christian and secular academics, (and some Muslims such as Abbas el-Akkad), as a late work and pseudepigraphical. With its obvious anachronisms and historical errors (like sailing to Nazareth) and utter lack of early manuscript evidence and irreconcilable differences between it and any ancient manuscripts, it is seen as a fourteenth century attempt to read later Islamic doctrine into the Bible, which clearly refutes it.   
For more information, see The Bible versus the Qur'an
Verses of violence
The Koran contains scores of verses exhorting Muslims to engage in violence against non - Muslims:
- "I will cast terror into the hearts of those who disbelieve. Therefore strike off their heads and strike off every fingertip of them" (Sura 8 verse 12). 
- "Say to the Unbelievers, if (now) they desist (from Unbelief), their past would be forgiven them; but if they persist, the punishment of those before them is already (a matter of warning for them). And fight them on until there is no more tumult or oppression, and there prevail justice and faith in Allah altogether and everywhere; but if they cease, verily Allah doth see all that they do" (8:38,39).
- "So when the sacred months have passed away, then slay the idolaters wherever you find them, and take them captives and besiege them and lie in wait for them in every ambush, then if they repent and keep up prayer and pay the poor-rate, leave their way free to them" (Koran 9:5)..
- "The Jews call 'Uzair a son of Allah, and the Christians call Christ the son of Allah. That is a saying from their mouth; (in this) they but imitate what the unbelievers of old used to say. Allah's curse be on them: how they are deluded away from the Truth!" (9:30) .
- "Soon shall We cast terror into the hearts of the Unbelievers, for that they joined companions with Allah, for which He had sent no authority" (Sura 3 verse 151). This verse commands violence against Christians, who according to Mohammed 'joined Companions with Allah' and are thus infidels who must be killed, subjugated, or converted according to the Koran.
Christians see such commands and exhortations are being contrary to the teaching of the Bible, as in the Old Testament ordained violence was limited to a specific area and people, (Dt. 7:1; Ex. 17:16) while under the [New Testament the church did not, in doctrine or in practice, seek to physically rule over those without: (1Cor. 5:12,13), nor use physical violence against them, or in disciplining its members, (Jn. 18:36; Eph. 6:12; 2Cor. 6:1-10; 10:3,4) as it must depend upon spiritual power. The New Testament does however, uphold the just use of the sword by the established civil powers. (Rm. 14:1-7;2Pet. 2:13,14) Rather than the use of religious violence being a characteristic of Christian groups that take the Bible to be their ultimate authority, more often such go to what some consider extremes of pacifism, while the use of physical violence by a church to subdue spiritual enemies, or to enlarge its reign, is seen by many to be the result of a man or organization being exalted above the Bible.
Treatment of Christians and Jews
In several verses, the Koran advocates Muslim tolerance and peaceful coexistence with Christians and Jews. The Koran refers to Christians and Jews as "'Ahl al-Kitab", which translates to "people of the book." The Koran acknowledges that Christians, Jews, and Muslims all worship the same god (Allah is one and the same with YHWH) and derive their beliefs from the same Abrahamic tradition.
"Those who believe, those who follow the Jewish, the Christian, and the Sabian scriptures--any who believe in Allah and the Last Day, and work righteousness, shall have their reward with the Lord." (Koran, 2:062)
"Argue not with people of the book. Except by what is best, except those of them who act unjustly." (Koran, 29:046)
"There are those people of the book who believe in God...humbling themselves before God...for them is a reward with the Lord..." (Koran, 3:199)
"O you who have believed, do not take the Jews and the Christians as allies. They are allies of one another. And whoever is an ally to them among you - then indeed, he is of them. Indeed, Allah guides not the wrongdoing people." (Koran, 5:51)
"The Jews say, "Ezra is the son of Allah "; and the Christians say, "The Messiah is the son of Allah ." That is their statement from their mouths; they imitate the saying of those who disbelieved [before them]. May Allah destroy them; how are they deluded?" (Koran, 9:30)
Doubts over authenticity
In 2015 a carbon-dating of the oldest known Koran, the "Birmingham Koran", revealed that it actually pre-dated the prophet Mohammed, which would obviously have tremendous implications for the religion of Islam. While it has not yet been fully confirmed, Saudi scholars have immediately raised doubts about the manuscript that was tested.
- See American Heritage Dictionary, Merriam-Webster, Webster's New World College Dictionary, and Oxford. For a comprehensive list of dictionaries, see OneLook.
- The Chicago Manual of Style, Section 8.102, "Koran; Koranic (or, less commonly Qu'ran; Qu'ranic)." The New York Times Manual of Style and Usage also gives "Koran" (p. 183).
- "Quran or Koran?" , American Journalism Review, 2006.
- The AP Stylebook gives Webster's New World College Dictionary as a "first reference for spelling" while Merriam-Webster is given as "second reference."
- Koran, by Gabriel Oussani, The Catholic Encyclopedia
- Science Encyclopedia, Sacred Texts - Koran - The Koran And Previous Scriptures
- answering-islam.org; Contradictions in the Qur'an
- Muhammad Versus Jesus Christ
The DEITY of CHRIST
- Kurt and Barbara Aland, 1987:82-83
- Jochen Katz, My Questions to Muslims; Answering Islam
- Texual History of the Koran
- Textual answering-islam.org; Variants of the Qur'an
- The New Covenant means of warfare