There have been many great books and pamphlets written on this subject with overwhelming citations, research, and results. Since there is so much out there that can explain in a greater way, I will only write a little summary of what I have learned from many books and historical research. Here is a great and detailed start: "Sharing Your Faith With a Muslim" by Abdiyah Akbar Abdul-Haqq. Bethany House Publishers.

Muhammed and essentially all of his followers were illiterate and uneducated. The knowledge that they had about the Taurat (Torah), Zabur (Psalms), and Injil (Gospel) mostly came from the Arabian Jews, who were not too devout themselves. These Jews were proselytized from the devout Jews in other parts of the world, and had not much rearing. They were not true Hebrews. These Arabian Jews were very alienated from their Scriptures in knowledge. The surrounding areas of pagans and other various beliefs dilluted the Arabian Jews' devotion to their pure religion.

Those who went to the synagogues had to have the Bible stories orally transferred to them, for the Bible and the stories were in Hebrew, while they only knew the native language, Arabic, Aramaic. The people who translated orally the Scriptures usually added in their own interpretations and extra details. Over time, the oral traditions which were spread by the Arab Jews to each other were put into writings called Targums. These were what the Arabian Jews used along with mystic traditions and rabbinic writings. They usually erred from the original Hebrew Scriptures greatly.

The Arabs that were searching for enlightenment and the religion of Abraham had to go through these Arabian Jews which made copies of their arabic targums to sell to the requesters. When they made these copies, they usually added in more mysticism and weird revelations which made them more interesting, and therefore, more "sellable." Thus the 'words' and 'scriptures' that the Arabs had were more than thrice-removed from the originals by undevout and money lustfull translaters. These targums are obviously what Muhammed talks about in the Koran:

[Sura 2:78] "And amongst them are ignorant persons, who know not the Book but only foolish stories; they follow naught but their imaginations."

[Sura 2:78,79] "Among them are unlearned folk who know not the Scripture except form heresay. They but guess. Therefore woe be unto those who write scripture with their hands and then say, "This is from God," that they may purchase a small gain therewith. Woe unto them for that their hands have written, and woe unto them for that they earn thereby."

[Sura 3:78] "Most surely there is a party amongst those who distort the Book with their tongue that you may consider it to be (a part) of the Book, and they say, It is from Allah, while it is not from Allah, and they tell a lie against Allah whilst they know."

He doesn't say that God's word changed, but that the Jews changed it with their tongues, orally, then wrote down stuff that is not of God. So what is left is written junk that is not scripture at all.

The many Christian dissentions (Coptics, Greeks, Nestorians, Eutichians, and Monophysites) caused a lot of confusion to those seeking truth. Many Arab Christians and those Chrstians in proximity to the travels of Muhammed had a mixture of orthodox y with heterodox beliefs and cultic ideas and rituals. It was a mixture of apocryphal and legendary along with orthodox. The effect of these inaccurate beliefs shows up when reading the Koran. Muhammed thought that Christians believed in three seperate gods: Jesus, his mother Mary, and God the father (Sura 4:171; 5:116). This is obviously not what Christians believe as one can see when one reads what the Bible says. Yet, there was an heretical sect of Christianity that beleived in Mary's divinity. If Muhammed was able to read what the Bible said, surely he would not have gotten his Judaic and Christian theology mixed up. There are many stories in the Koran that come straight from apocrypha (fake books that are not of God) books of the Bible, the books that heretical sects wrote (i.e. the storie about Jesus giving life to a clay bird). Muhammed didn't know the difference, for he didn't have a true Bible to check it with. This means that Muhammed, using the the fake books and stories that people made (or the god that Muhammed got them from), believed lies and falsities, and then called it the perfect revelation of God, the Koran. We know that many of these aprocryphas ARE fake and not accurate historically or textually. This means that Muhammed was not writing the truth when he (or his god) put these stories into the Koran.

For instance, the Koran says that Mary's father was Imran. The Bible gives no knowledge of the father of Mary, the mother of Jesus. However, the Bible says that the father of Miriam, is Amram (Imran when traslated to Arabic). Amram was the father of Miriam, Moses, and Aaron, according to the Bible. The Muslims call the mother of Jesus Mariam, and the father of Mariam Imran, and the brother of Mariam they call Aaron. Muahammed got Mary mixed up with Mariam of the Old Testament. Many Muslims try to explain this discrepencey away by saying that God preserved Mariam so she could give birth to the prophet, Jesus. It seems that when Muhammed wrote the Koran he couldn't remember all the details of the stories that the Jews and the Christians told him. For the Koran can't be of God (Yahweh) when the stories are all mixed up. There are many, many more examples of names, time-frames, and locations that are all mixed up and deviate from the Bible's stories. Many of these cause the whole attituded and ideas of the stories to change. They are not just small discrepancies, especially when book (the Koran) claims itself to be perfect.

The Bible was inaccessible to Muhammed and his followers, and so, they were ignorant of what was in it, except from stories told around the fire and from others who were also ignorant. That is why Muhammed could not check his revelations with those of the Bible and see if they lined up, and were actually from God. From the stories that he had heard and from encouragement from an ignorant Christian family member, Muhammed thought he was the next prophet, and so went on spreading his 'revealed word'.

Muhammed at first thought he was possessed by a demon and that his revelations were of an evil spirit. Kadija assured him that he was called of God. Yet, Muhammed was cautious and even so depressed and unsure until he wanted to just end the pain and kill himself from a cliff's edge. Muhammed was searching for the religion of Abraham, and believed that it was the truth. That is why he believed the Bible - even though he hadn't read it - because it was from the God of Abraham. That is why he told his followers to believe the Bible and check his revelations up with it. He said, "If you are in doubt as to my word, then go and ask the people of the Book, for they know the truth." The people that Muhammed told this too didn't have access to the true Bible either. That is why they didn't oppose him. However, there were a few people that did know the Bible eventually, and they opposed Muhammed, and either forsook Islam or never embraced it. Such were: Warqa b. Naufal (cousin of Muhammed's first wife; Ubaidullah b. Jash, followed Muhammed with first band of Muslims inMecca for a while, but then forsook it for Christianity; Uthman b. Al-Huwayrith, relative of Kadijah also, became Christian and opposed Islam. Others learned in the Scriptures rejected Muhammed's revelations. This brewed the hatred towards the Christians and especially Jews from Muhammed.

Too bad Muhammed couldn't check his new revelations up against the true word of God (and chose not to in after gaining so much power and wealth for fear of loosing it because of personal guilt. For if he had, he surely would have repented and sought help for his evil spirit.