In comparing Christianity and Islam we must compare the teaching of the Bible with the Koran. The Bible is the Holy Book of the Christians, and was completed in the fist century AD. The Koran is the Holy Book of the Muslims, and was completed in the seventh century AD.

But the Koran also says:

• Muslims should not be friends with Christians or Jews. (Who do not believe in the oneness of God) Surah 5:51
• Christians who believe that Jesus is the Son of God are like pagans and Allah's curse is on them. Surah 9:30
• Jesus never commanded us to worship him or his mother Mary. Surah 5:116
• Jesus was not crucified, it just appeared so. Surah 4:157
• There is no trinity. Surah 4:171
• The revelations given to Mohammed are to be judged by the "book" or the Bible. Surah 10:94

In many places in the Koran, Christians are referred to as the "people of the book"
"And in their footsteps we sent Jesus the son of Mary, confirming the Torah that had come before him: We sent him the Gospel: therein was guidance and light. And confirmation of the Torah that had come before him: A guidance and an admonition to those who fear Allah. Let the people of the gospel judge by what Allah hath revealed therein." Surah 5:49-50
So just as the early Christians judged the New Testament by the Old Testament so modern Christians are to judge the Koran by the Bible.

All the prophets in the Bible proved themselves by miracles and prophecies. Muhammad was unable to perform miracles to prove he was a prophet. Surah 29:50 The only prophecy recorded in the Koran is that Allah will make the Christians victorious over the pagan Persian Empire. Surah 30:2-4 Jesus performed numerous miracles, was a prophet born of the virgin Mary Surah 3:47, 9:20-31, and was taken up to heaven Surah 3:54, 4:158. Muhammad never did any of these things and yet he is considered the last and greatest of the prophets. Surah 33:40

The Bible declares the Gospel to be:

Jesus in the gospels, Peter, Jude, and Paul warned about false teachers and prophets that would teach things that are not true. Paul in Galatians 1:8 says if an angel from heaven, should preach to you a gospel contrary to that which we have preached to you, let him be accursed.

There is only one God, but the father, the son, and the Holy Spirit are all called that one God. This is defined as a trinity. It is not polytheism. For a detailed explanation of the trinity see: The Deity of Christ; and Cults and the trinity; at http://www.biblefacts.org. 

Although there have been textual changes to the Koran and the Bible through the centuries, the basic story has not been changed. All copies of the Koran say Jesus was not crucified and there is no trinity, and all copies of the Bible say Jesus is God incarnate and that he did die on a cross to pay for our sins and reconcile us to God the Father.

There is no evidence that these teachings in the Koran have been changed. The oldest copy of the Koran is dated about 790AD, although it is not available to the public.

There is overwhelming evidence that the teachings in the Bible have not been changed, over 5300 manuscripts, some going back to the first century, and the Dead Sea Scrolls, just to name a few, show that the new testament (Ingeel) is exactly the same as it was in the first century. Whole Bibles, called codices, dating all the way back to 350AD are available in museums around the world. That's 250 years before Muhammad.

In conclusion, the Gospel of John states that in order to be saved we must believe Jesus is God, and that he died on a cross. The Koran says the opposite. So if the Koran has not been altered, and the present day Gospel of John is the same as it was in the first century, then the Koran must be a false writing. The only possible way for the Koran and Islam to be true would be if another gospel of John were found. One that agreed with the Koran. Then the debate could begin. Until then the issue is seems settled.

APPEDIX
There are many prophecies in the Bible, there are just a few that pertain to our discussion:

This prophecy was given to Daniel c500bc. The decree referred to here was given March 14, 444 BC. See Nehemiah 2:1. To figure the exact date of the cutting off (Death) of the Messiah (Jesus) we need to convert the Jewish calendar to our own Roman calendar. 483 years times 360 days of the Jewish year equals 173880 days. 476 Roman years times 365.25 equals 173859 days, a difference of 21 days. So 444 BC plus 476 years equals 31 AD. Add one year, because there is no zero year on our calendar, and that comes to 32 AD. March 14 plus 21 days equals April 6. The 69 weeks ended April 6, 33 AD. The exact day Jesus was crucified.

In Daniel 5, there is a story of God's hand appearing and writing a prophecy, Mena Mena Tekel Uparson, on a wall in Babylon. The prophet interprets the writing on the wall to be starts by reminding Belshazzar of the 'seven times' of Nebuchadnezzar. Since Belshazzer was rebellious against the LORD, his kingdom will be taken that very night by Cyrus who would begin to free the Jews. Scholars have long noted that the writing was made of words that also had a numerical meaning in the Babylonian language. The Mena was 1000 Garahs (basic monetary value), the Tekel was 20 Garahs, and the Pares was half a Mena or 500 Garahs. If we add these up we come to 2520. This is another year to day prophecy for Israel again. We calculate the date just like we did in Daniel 9.

2520 years on the Jewish calendar times 360 day is 907200 days. That divided by 365.242 days is 2483.83 years on our calendar. The Jews returned to Israel after the 70 year captivity in the spring of 536 BC. 536BC. plus 2483.83 years, plus the one year for the calendar change is 1948, which is the birth of the modern state of Israel. The local people prevented the Jews from rebuilding the temple until the final decree came in 517 BC. from Darius. And of course 517BC. plus 2483.83 plus one year for the calendar change is 1967. Which was the year the Jews took back the temple mount. But they still had trouble from the local people, so it took another 100 years or so more before they had a completed temple.

The Bible tells that the next Jewish Temple will be rebuilt before the Second Coming of Jesus in 2 Thessalonians 2. Currently the dome of the rock sits on the temple mount preventing the rebuilding of the entire Jewish temple.

In Revelation 11:2 we are told the next temple will be built without the outer gentile court. This will be part of the peace treaty that the antichrist makes with Israel and allow both the dome and the Temple to sit on the temple mount for 42 months. Ezekiel 42:20 tells us there will be a wall between the new Holy Temple and the profane (or common) thing that sits in the south.

Even the Koran tells us Allah ordained the Holy Land for the Jews. Surah 5:24.
So we can plainly see it was Gods will to punish the Jews with exile in 70AD. And to bring them back to the land of Israel as rulers in 1948. To take control of the Temple Mount in 1967. And anyone not submitting to the Jewish government is fighting against Allah himself. Just like anyone who tries to blow up the Dome of the Rock on the temple mount.

For more information See: http://www.answering-islam.org