First of all, even though it is common to say that the Quran confirms the Bible, strictly speaking this is not correct. The term Bible does not appear anywhere in the Quran. The term Old Testament and New Testament does not appear anywhere in the Quran. The Quran actually confirms the original revelation that was given to Prophet Moses called the Tawrah (Torah) and the Enjeel (the Gospel) that was revealed to Prophet Jesus. Other scriptures that are mentioned in the Quran include the Zabure revealed to Prophet David and the Suhuf revealed to Prophet Abraham. The idea that the Quran confirms the Bible, the Old Testament or the New Testament is incorrect. Even then when we take a term like Torah, it isn’t the exact equivalent in understanding the scriptures between Muslims and Jews and Christians, for example. Among the Jews and Christians the Torah is believed to be the first five books, beginning with Genesis, in the Bible.

However, if you look carefully into these books, you’ll find many of them don’t really represent revelation given to Moses but are biographies of Moses. Also, towards the end of chapter 34 in the book of Deuteronomy, which is part of the Torah it talks of Moses’ death and being buried, which obviously is not of the work of Moses nor is it the revelation given to him on Mount Sinai as Muslims believe. As such even the definition of Torah in the Judea-Christian literature is not like the Quranic reference to the Torah, or law, specifically the revelation given to prophet Moses not biographies about him.

Secondly, the term Enjeel, in the Quran, the equivalent of the Gospel (in the singular form) should not be equated with the four Gospels. The Quran speaks of the word of God, not the word of Mark, Luke, Matthew, and John. That is not the word of God, that’s their own biographies. What the Quran speaks of is the revelation given to Prophet Jesus, peace be upon him, something that he was guided by divine revelation. Whether he asked people to write it or not we don’t know for sure, but it is the same type of divine revelation that was given to Moses, Mohammad, Abraham, or David for that matter, may peace be upon them all.
I’d like to raise another issue as well. When the Quran speaks of confirming any previous scriptures, it is conditional and indicates in no uncertain terms that the Quran and the Quran alone as the last well preserved revelation is the final judge and the criterion to sift through any previous scripture to discern what is the word of God and what is the word of humans; which parts remained intact and which parts might have gone through some changes throughout history. The term muhaymen, which appears in the Quran, in surah number 5 and verses 48 through 51, deals specifically with this issue of the Quran being muhaymen. This word, muhaymen in Arabic, as Mawlana Mawdudi explains in his Commentary on the Quran, means to uphold, to safe guard or preserve, to watch over and to stand witness. All of these definitions apply to the Quran in its relationship to previous scriptures. First of all, the Quran safeguards and preserves the teachings of previous prophets. It watches over the revelations that God sent before by explaining their true meanings to negate any confusion, misunderstanding or misinterpretation that has arisen throughout history. It stands witness because it bears witness, as Mawdudi says, to the word of God contained in those previous scriptures and helps sort it out from interpretations and commentaries that were later added to them.
The third issue is that some people would say that the Quran itself says that there is no one who will change the word of God. And so how could Muslims say that the Bible has changed from the original revelations given to these prophets? Now if you refer to the Quran and see what some of those writers refer to, you’ll find that there are only three verses in the Quran that speaks about changing the word of God. Each one of them appears in a different meaning depending on the context of the surah. I also checked the tafseer, interpretation of the Quran.
First of all, in surah 6 verse 115, kalimat or words as it is often translated to say, is used in the sense of decree that no one is going to change the decrees of God in creation. In the same surah but in verse 34, kalimat is used here in the sense of the promise of God, when read in context, to give victory to His messengers. In surah 18 in passage 27, the word kalmiat appears in the sense of preserving God’s words or creation. Notice here that the promise made that His words will be preserved does not cover the promise to preserve the words of human beings. Some biblical scholars, for example, raise the issue that we don’t know whether John wrote this or not or Paul wrote this or not etc. Another example: is the book of Hebrew actually written by Paul or someone else? That does not go within the promise of God, because they are the words of humans and not the word reveled by God to His messengers and prophets like Moses, Jesus, or Mohammad peace be upon them.
Secondly, in any religion that says that no one can change the word of God, we have to look at it on two levels. In any religion, anyone can change the word of God on paper. One can get a copy of the Bible and write it out differently. One can get a copy of the Quran and change it. So the physical change in terms of writing, any human can do that in any religion for that matter. But the level that the Quran refers to, even when it speaks about the revelation, that no one is going to change the word of God. It means the essence of His revelation will ultimately be preserved and would be protected from change. Even though people may have changed or attributed words to God that He didn’t say, or people have forgotten or lost part of the scriptures ultimately it will be preserved. And Muslims believe that this is precisely one of the great benefits of the Quran as the last revelation, which has been totally protected, that restores and clarifies the word of God that was given to different prophets because we believe in the unity of the mission of all of these prophets. In this sense, there is the promise that the word of God was ultimately preserved.
Interestingly enough the Quran gives good criteria to find out which book can be judged as being the word of God in its totality. This appears in surah 4 verse 82:
“Then do they not reflect upon the Quran? If it had been from [any] other than Allah, they would have found within it much contradiction.”