The Qur'an talks of the Injil ( إنجيل ) referring to the New Testament of the Bible - usually the four Gospels
The Qur'an reports the Injil to be one of the three previous 'Revelations' of Allah (the other two being the Taurat and Zabur). The Qur'an asserts that Allah revealed the previous scriptures to the Jews and to the Christians, but that those who knew the scriptures 'changed the words from their right places' and 'forgot a good part of the message'. Regarding the Injil:
And with those who say, We are Christians, We made a covenant, but they neglected a portion of what they were reminded of, therefore We excited among them enmity and hatred to the day of resurrection; and Allah will inform them of what they did.
Qur'an 5:14
The Qur'an also claims to 'confirm what they (People of the Book - Jews and Christians) have with them':
O followers of the Book! indeed Our Messenger has come to you making clear to you much of what you concealed of the Book and passing over much; indeed, there has come to you light and a clear Book from Allah;
Qur'an 5:15
Muslim Claims about the Injil
Despite the clear words of the Qur'an, the majority of Muslims errantly believe that Allah is telling them that the "People of the Book" have corrupted their books. Muslims proclaim that the actual texts have been changed; that not many of Allah's original words remain in them. The Qur'an however never charges this against the "People of the Book". It clearly states that The Jews and Christians knew what their texts said, but that they deliberately distorted the commands and meanings in their oral recitations to the illiterate people; that they were saying "my book says to do this" when the text on the page did not. The Qur'an also charges that they 'concealed' a part of the message or 'hid it (the book) behind their backs.'
To keep their claim of textual corruption of the 'previous scriptures' afloat, Muslims will often present Qur'an 2:79 as evidence of their interpretation. However, viewing this verse in context, juxtaposed against other Qur'anic verses proves this interpretation to be incorrect.
Muslims also claim that the Injil is corrupt because of the differences in the accounting of events. The most significant difference is that in the New Testament of the Christian Bible, Jesus is believed to be God; yet the Qur'an says Isa (Jesus) was just a prophet, and that Allah has no partners. This error can be explained by Muhammad's inability to understand the concept of the Christian Trinity (that there is one God, existing in a unity of three persons: Father, Son, and Holy Spirit).
Why does Allah protect his Qu'ran, and not his Injil and Taurat?
Although Muslims believe the 'previous scriptures' to be corrupt even though they were revealed by Allah, they believe the Qur'an remains unchanged from the moment of Revelation, until this day; that Allah is 'protecting it from corruption.' Muslims are also told that they are to believe in the "previous Revelations", although the word of the Qur'an supercedes them all. This does not fit with the claim of textual corruption.
So the question remains: Why does Allah protect the Qur'an, but was unable to protect the Injil? Also, why would you be ordered to believe in something that you claim is corrupt?
What was Isa's "Good News"?
Allah claims to have revealed the Taurat (the Law) to Musa (Moses), the Zabur (Psalms) to David, the Injil to Isa for the Jews and Gentiles (Christians) and the Qur'an to the Arabs. "Injil" is the Arabic word for the Greek word "Evangelia" which means Good News. Allah confirms in the Qur'an that the Injil was given to Isa, so this raises a big question:
If each Revelation from Allah was a confirmation of the scripture that came before it, what was the Good News that Isa came to share? If it is a confirmation of that which the Jews already had with them, then he would not be bringing anything new; so there is no reason to call it Good News. Surely the Qur'an would be the only Good News in the history of the Revelations because it was (allegedly) brought to us by the Final Messenger and was the Final Revelation of Allah?
"Injil" is a Christian, not an Islamic concept. "Good news" should be exactly that; good news. In Christianity, people need to be saved, and they do so through accepting Jesus Christ as their Saviour. However, according to the Qu'ran people have no need to be saved. According to the Qu'ran, people are only doomed if they receive the "signs of God" and choose to reject those "signs". So what was Isa's "Good News"?