It is difficult to underestimate the prevalence of the argument that the Bible has been corrupted among Muslims today. Not only is it a common view, but with so many Muslims appealing to the testimony of the Quran to prove the Bible has been changed, you would think that this is something that Muslims would have believed from the 7th Century.
It may be a surprise then to learn that the Muslim view of corrupted scripture only began to emerge 400 years or so after Muhammed. Nowhere in the Quran or Tavsir (the early commentaries on the Quran), the Sira (the biography of Muhammad) or in the Hadith is the explicit claim that scriptures of the Jews and Christians have been corrupted.
The first Muslim scholar to make popular arguments that the Bible has been changed was a Spanish scholar called Ibn Hazm (wrote 1056 AD) followed by al-Juwayni. What drove them to develop their arguments was the fact that it was only by then that translations of the Bible were becoming available in Arabic, and they realised that the Bible does not actually mention Muhammed as the Quran states in Quran 7.157. The fact that they could see that the text of the Bible and the Quran contradicted each other drove them to conclude that the earlier scriptures had become corrupt.
These arguments still prevail today and leave Muslims in a contradiction; on the one hand the Quran claims that the previous scriptures are trustworthy, yet on the other hand Muslims hold the view that the Bible has been corrupted.
Two Quran verses that are useful to help friends see this contradiction are Quran 10.94 and Quran 21.7, which command Muhammad, if he is in any doubt about the revelation given him by Allah, to “go to the people of the book” and get answers from them.
The “corruption of scripture” argument is often a default objection in response to many truths of the Christian faith and therefore it is important to take time to expose the lie our Muslim friends believe. Point out that Satan has sought from the beginning to question what God has said, and that it is no different today. Most Muslims have never explored the facts or lack or facts behind the claim that the Bible has been changed, and blindly accept what they are told. Encourage them to investigate it for themselves and to take the time to read the New Testament prayerfully.