Many Christians accept the Muslim claim that we both worship the same God. They claim that they call him Allah, while we call him God. It is not unusual to hear Christian leaders make such statements. Bible societies have even gone so far as to use the name Allah in the Bibles they produce for Arab Christians.
The problem with this is two-fold. First, history and archeology show clearly that Allah was worshipped as a pagan moon god long before Mohammed came on the scene. Robert Morey, author of "The Islamic Invasion," explains:
"Islam's origins have been traced back by scholars to the ancient fertility religion of the worship of the moon god which was always the dominant religion of Arabia. The moon god was worshipped by praying toward Mecca several times a day, making an annual pilgrimage to the Kabah which was a temple of the moon god, running around the Kabah seven times, caressing an idol of a black stone set in the wall of the Kabah, running between two hills, making animal sacrifices, gathering on Fridays for prayers, giving alms to the poor, etc. These were pagan rites practiced by the Arabs long before Muhammad was born."
"What religion today practices the pagan rites of the moon god? Islam! This explains why the crescent moon is the symbol of Islam. It is placed on top of mosques and minarets and displayed on hats, flags, rugs, amulets and even jewelry. Every time you see the Muslim symbol of a crescent moon, you are seeing the ancient symbol of the moon god."
Second, if you read the Qur'an's description of Allah, and read the Bible's description of God, it becomes obvious you are reading about two different persons. Allah orders his followers to kill those who deny Islam, while God instructs us to love our enemies. Allah had no son while God sent His Son to die for sinful men. Allah is "unknowable" while God seeks a personal relationship with His creation, man.
The spirit behind Islam is an entirely different spirit... a spirit that denies the deity of Jesus Christ. Any Christian who accepts the notion that Allah is God creates an impossible situation. Since the Qur'an contains our only revelation about Allah, they will be forced to look there as their authority. The Qur'an specifically denies the deity of Christ! All Christian witness ends right there.