MUHAMMAD HAD REVELATIONS THAT HE WROTE DOWN AND SAID THEY WERE DICTATED TO HIM BY THE ANGEL GABRIEL. However, Muhammad was supposed to be illiterate and not capable of writing. Many people question that Muhammad was visited by the angel Gabriel and accept that he was really a Roman Catholic plant to rein in true Christians but the plan backfired.
Many people question whether Muhammad was actually given what he wrote from the angel Gabriel. The reason why they question this is because, on the one hand, Muhammad says that we are to believe in what is written in the Bible and, then, on the other hand, he resorts to what appear to be devilish actions that are contrary to what Jesus taught concerning loving one’s neighbor, forgiving other people, and leaving vengeance to God alone.
From an observation of what goes on among Muslims, vengeance is something that they seem to major in. From a Biblical perspective it is clear that vengeance belongs to the Lord God, Creator of the Universe.
Vengeance is mine, and recompense, for the time when their foot shall slip; for the day of their calamity is at hand, and their doom comes swiftly. (Deuteronomy 32:35)
Beloved, never avenge yourselves, but leave it to the wrath of God; for it is written, “Vengeance is mine, I will repay, says the Lord.” (Romans 12:19)
For we know him who said, “Vengeance is mine, I will repay.” And again, “The Lord will judge his people.” (Hebrews 10:30)
The Ex-Jesuit Priest Alberto Rivera — a man who was ordered to be murdered because of what he was revealing—says that the Vatican sponsored Muhammad as an orphan to be raised by a woman in Mecca in order to create a means by which the Roman Catholics can subdue the Christians in those lands that rejected the Pope as the Vicar of Christ on Earth.
Now when people take into account what happens among Muslims, how they fight among themselves (for instance, the Iraq-Iran war), what a man like Alberto Rivera reveals, and what the Bible states about God’s purpose on Earth, major questions arise concerning Muhammad’s legitimacy as a prophet of God.
Muhammad claims that Gabriel appeared to him and dictated the Quran in some cave. When we read the Quran, this seems like a story of two adventures: the adventures of Muhammad in Mecca and the adventures of Muhammad in Medina. When in Mecca Muhammad has a softer message than that what he espoused in Medina; a message more in line with what we might expect from a prophet.
Dr Bill Warner has spent over a decade dedicated to studying Islam. He writes:
Statistical methods applied to the Islamic texts showed that:
Islam is far more of a political system than a religion.
There is no unmitigated good in Islam for the Kafir (non-Muslim).
Islam’s ethical system is dualistic and is not based on the Golden Rule.
Islamic doctrine cannot be reconciled with our concepts of human rights and our Constitution.
The great majority, 96%, of all Islamic doctrine about women subjugates them.
The Sunna (what Mohammed did and said) is more important than the Koran in a Muslim’s daily life. 
Muhammad’s father died when he was six months and his mother died when he was six years old. From Wikipedia we learn that Muhammad could have been influenced by Bedouins, his paternal grandfather Abd al-Muttalib, his uncle Abu Talib, and the woman he married. 
In having said the above, it is possible that Muhammad may have had the angel Gabriel appear to him. For there is a prophecy in the Bible that indicates that something was to happen of significant importance in respect to world affairs, and therefore the plan of God, about the time Muhammad came on the scene.
Unfortunately, for Muhammad, he deviated from the straight and narrow instead of doing what was intended. However, unlike Abraham, God foreknew that Muhammad would choose to become a political operative rather than one who would seek the Kingdom of God—something that is not of this Earth.
Notice what the Bible states about the Kingdom of God and Abraham.
Neither shall they say, ‘Lo, it is here!’ or ‘Lo, it is there!’ For behold, the Kingdom of God is within you. (Luke 17:21)
As it is written, “I have made you [Abraham] the father of many nations”—in the presence of the God in whom he [Abraham] believed, who gives life to the dead and calls into existence the things that do not exist. (Romans 4:17)
And He [God] brought him [Abraham] outside and said, “Look toward heaven, and number the stars, if you are able to number them.” Then he said to him, “So shall your descendants be.” And he believed the Lord; and he reckoned it to him as righteousness. (Genesis 15:5–6)
True religion is about the quest to unite with God and establish eternal security now. Religion is not about politics or ruling over others and subjugating them. Religion is concerned about finding the truth and overcoming death.
False religion is just a means to control people. In this respect, it appears that Alberto Rivera may be right, and Islam is the result of a failed attempt by the Roman Catholic Church to bring the Middle East under their control. Whatever the case, this was foreseen by God Almighty.
Muhammad could have been influenced by Roman Catholics, the Bedouins, His uncle, His wife, by people in Mecca and Medina and, maybe, by the angel Gabriel. Muslims are indoctrinated to believe that Muhammad received revelation from the angel Gabriel. Non-Muslims cannot understand why God would depart from the message of love and forgiveness and that He will judge everyone by Lord Jesus Christ, whom Muhammad said did not die and shall return at the last day.