How do we know Islam is a false religion?
Can God preserve His word? Yes or no.
No. Why would we expect that the Quran is preserved if God couldn't preserve prior revelations? If God cannot preserve His word we cannot Attribute any scripture to God including the Quran. If He cannot preserve His word He is not God. Therefore Islam is a false religion.
Yes. Then we have 2 options either He can but didn't or He did. If the Bible was corrupted and God did not prevent it then God is not Great as there is a time frame of about 500+/- years where followers could not have the truth in order to be saved. Since both Muslims and Christians believe God is great then He can and would preserve His word.
If this is true then we should default to the oldest scripture. According to Islam the Torah and the gospel are from God and are older than the Quran.
"The Bible was fully written and confirmed and compiled by the Lord 500 years before Muhammed was even born. Sura 6:115 (AI-An'am) says: "Perfected are the words of your Lord in truth and justice. None can change them. He is the Hearing, the Knowing." The Torah and the Gospel are the word of the Lord, according to the Quran, so no one can change them!"
Sura 5:68 (Al-Maida) - "Say: 'People of the Book (Christians and Jews), you stand for nothing until you observe the Torah and the Gospel and that which is revealed to you from your Lord."
Sura 4:136 (Al-Nisa) - "0 believers, have faith in Allah and His Apostle, in the Book He has revealed to His Apostle, and in the Book He formerly revealed (Bible). He that denies Allah, His angels, His Scriptures, His apostles, and the Last Day, has strayed far from the truth."
Sura 5:48 (Al-Maida) - "And to you We have revealed the Book with the truth confirming what was revealed before it in the other Books, and standing as a guardian over it.")
The Bible and Quran fundamentally and theologically contradict each other, they are mutually exclusive. Therefore there is no way to reconcile the Quran and the Bible. If God can and does preserve His word then we should logically go to older manuscripts and disregard later ones that contradict earlier ones.
Saying the Bible was corrupted is a very poor way to reconcile the contradictions. Why did Mohammad neglect to mention in the Quran and Hadith that the bible was corrupted? If it happened before the Quran surely Allah would have revealed that to Mohammad to include in the Quran or Hadith. If the Bible was corrupted after the Quran was revealed why was the Quran necessary? Either way we have biblical manuscripts of the Bible that are much older than the Quran.
If the Bible is false the Quran is false because the Quran affirms the bible yet contradicts it. If the Bible is true the Quran is false because they are mutually exclusive. Either way the Quran is false. The Bible is much the much older text therefore if God is great the Bible is true and the Quran is false.
For further reading:
http://www.churches-of-christ.net/tracts/job083u.htm