While Islam refers to Christians as "people of the book" along with Jews and Zoroastrians (as opposed to polytheists and animists), it denies that Christianity is the final and permanent revelation of God and speaks quite negatively of Christians as "the infidels" (kaffur). This is because Muhammad, "the seal of the prophets," is the last and greatest of the prophets of Allah (Qur'an 48:27-28). He alone corrects the errors of the past, including the aberrations of Christianity. Islam abrogates Christianity (Qur'an 48:27-28); it is Christianity's replacement. The argument for abrogation is rooted in five major claims made by Islam against Christianity. This is a significant apologetic challenge that Christians today need to face intelligently, given the global reach of Islam and its growing influence in the West.
Claim # 1 The original holy book has been distorted. Since there are distinct differences between Christianity and Islam concerning the nature of God, humans and salvation (as well as devotional practices), Muslims need to account for these discrepancies while affirming that Moses, David and Jesus were bona fide prophets of Allah. Therefore, Muslims charge that the original revelation to the Jewish and Christian prophets (who were all prophets of Allah) has been altered and distorted. This charge takes two forms. Either it is claimed that (1) the writings of the Bible even in their original form were distorted, or (2) the original Christian documents supporting porting Islam were tampered with after the fact. The second claim provides the better argument for the Muslim since the Qur'an endorses the divine authority of the Old Testament and the Gospels (Qur'an 4:48, 136; 5:47-51, 68-71; 10:94).
But … the New Testament has been transmitted with integrity. Moreover, the Old Testament texts from Muhammad's day are substantially similar to what we read in our Bibles today. The claim that Jews or Christians had radically changed the original documents is logically unsupportable. First, the magnitude of the change would have been enormous. All references from the Bible that contradict Islam would had to have been inserted into the documents. That would have included the Trinity, the incarnation, the crucifixion of Jesus, salvation through faith alone and more. Second, given the rapid dissemination of the New Testament in the ancient world, it would have been impossible for any group to seize and alter all the texts at hand. Third, we know of no early Christian texts of the New Testament that lack these distinctively Christian doctrines.
Islam faces another obstacle in establishing its accusation that the Bible has been radically altered such that it no longer teaches the truths of Islam. The Qur'an, supposedly received from Allah by Muhammad from 610 to 632, tells the reader to consult the Christian Scriptures to corroborate the veracity of Muhammad's message and his status as a prophet.
But if you are in doubt as to what We have revealed to you, ask those who read the Book before you; certainly the truth has come to you from your Lord, therefore you should not be of the disputers. (Qur'an 10:94; see also 5:47-51, 72; 19:29-30; 21:7; 29:46-47)Gleason Archer summarizes the Qur'an's teaching by noting first that "the author of the Quran firmly believed in the full inspiration of the Old Testament and the Gospels of the New Testament as containing the authoritative Word of God, and secondly that the Hebrew-Christian Bible should be appealed to in confirmation that what is revealed in the Qur'an is the very truth of God." Yet manuscripts of the Bible from this time period are in substantial agreement with what we read in our Bibles today. Archer observes:
It is completely out of the question to discredit the text of Holy Scripture as no longer conforming to what was current in Muhammad's time, from A.D. 610-632. Complete manuscripts of the New Testament copied out in the fourth century (Codex Vaticanus and Codex Sinaiticus) and the fifth century tury (Codex Alexandrinus), antedate the revelation of the Qur'an by three centuries.Therefore, when the Qur'an says to consult the Christian Scriptures for the verification of the truth of Islam, it contradicts itself. The extant Christian Scriptures of Muhammad's day teach that God is a Trinity, that Christ is the incarnation of God, and that salvation is through faith in Jesus Christ-all doctrines that Islam rejects.
Furthermore, Jesus, whom the Qur'an esteems as a prophet, endorsed the Old Testament as God's revelation (Matthew 5:17-20; John 10:35). He deemed his own teaching to be in accordance with that previous disclosure from God. Yet, as we will see, Islam denies crucial claims about Jesus.
Douglas Groothuis. Christian Apologetics: A Comprehensive Case for Biblical Faith (pp 604-606)