The Word of God Cannot Be Changed
When Christians use the word Bible, they are not merely referring to the New Testament, but to the Old Testament as well. This includes the Torah.
As the Quran (as well as the Bible) affirms, God's Word cannot be changed.
Perfect are the words of thy Lord in truthfulness and justice; no man
can change His words; He is the All-hearing, the All-knowing. S. 6:115
for them is good tidings in the present life and in the world to come.
There is no changing the words of God; that is the mighty triumph. S.
Recite what has been revealed to thee of the Book of thy Lord; no man
can change His words. Apart from Him, thou wilt find no refuge. S.
The grass withers, the flower fades, but the word of our God will stand forever. Isaiah 40:8 ESV
For truly, I say to you, until heaven and earth pass away, not an iota, not a dot, will pass from the Law until all is accomplished. Matthew 5:18 ESV
So, because we know God's Word cannot be changed, we know that we can have confidence in the accuracy and reliability of the Old and New Testaments.
Warnings from God's Word
Using, then, the Old and New Testament as a foundation, there are a few key points with regard to any new work that proclaims itself the Word of God.
God's Word warns that there are many deceivers who will attempt to distort the truth about God.
For many deceivers have gone out into the world, those who do not confess the coming of Jesus Christ in the flesh. Such a one is the deceiver and the antichrist. 2 John 1:7 ESV
God's Word warns against the preaching of any other gospel other than the gospel of Jesus Christ. In fact, it goes so far to say that even if an angel of God should preach a different gospel, we are to reject it.
I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a different gospel— 7 not that there is another one, but there are some who trouble you and want to distort the gospel of Christ. 8 But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed. 9 As we have said before, so now I say again: If anyone is preaching to you a gospel contrary to the one you received, let him be accursed. Galatians 1:6-9 ESV
The Quran's Affirmation of the Bible
The Quran itself affirms the Old and New Testaments:
Dispute not with the People of the Book save in the fairer manner,
except for those of them that do wrong; and say, 'We believe in what
has been sent down to us, and what has been sent down to you; our God
and your God is One, and to Him we have surrendered.' S. 29:46 Arberry
Then We gave Moses the Book, complete for him who does good, and
distinguishing every thing, and as a guidance and a mercy; haply they
would believe in the encounter with their Lord. This is a Book We have
sent down, blessed; so follow it, and be godfearing; haply so you will
find mercy; lest you should say, 'The Book was sent down only upon two
parties before us, and we have indeed been heedless of their study';
or lest you say, 'If the Book had been sent down upon us, we had
surely been more rightly guided than they.' Yet indeed a clear sign
has come to you from your Lord, and a guidance and a mercy; and who
does greater evil than he who cries lies to God's signs, and turns
away from them? We shall surely recompense those who turn away from
Our signs with an evil chastisement for their turning away. S.
So, whenever any other book is offered as another Word from God, we need to be very careful and "test the spirits to see whether they are from God, for many false prophets have gone out into the world." (1 John 4:1 ESV) Also, if any other book contradicts the Bible, then it is another gospel and must be rejected.
In the case of the Quran, it was actually claimed to be delivered to Muhammad by an angel of God. God's Word gives us a very specific warning about such a claim. The Quran contradicts the Bible and presents another gospel.
The Bible is God's Word--and God's Word cannot be changed. Therefore, we must conclude that the Quran is not the Word of God, and that is why Christians do not accept it as such.