At first glance, there is a great deal of common ground shared by Christianity and Islam. Both profess faith in one God, Jesus as the Christ, and the Bible as the inspired word of God.
But common ground quickly disappears. Unfortunately, Christians and Muslims do not always mean the same thing when they talk about believing in God or believing in Jesus. There simply is no reconciling Christianity and Islam. One or the other is false, or both are false, but there is certainly no way both can be true.
Who Is God?
I have heard some Christians and some Muslims say that we believe in the same God but just call him by different names. But Allah is not just another name, or even an incorrect name, for the God of the universe. Allah is an entirely different God.
The One God of the Bible is a triune God, comprised of the Father, the Son, and the Holy Spirit. Islam denies that there is a Godhood, and teaches that Allah stands alone.
Who Is Allah?
1. Allah was an Arabic god with a purely Arabic name, corresponding to the Babylonian Baal (various spelling - Bel) (Encyclopedia of Religion). It is not an Arabic word meaning deity. It is an Arabic proper name! Allah has an Arabian historical identity, but in connection with Abraham, Moses or any other Biblical character, he has no history at all.
2. Allah was a "high" god, the moon god, until Muhammad came along. He was but one of the pantheon of 360 gods that were worshipped by the pagan Arabs in the Kaaba (various spellings, such as Kabah or ka'ba), a temple in Mecca. Many encyclopedias make reference to Allah in this pre-Islamic period. He was the moon god, and he was married to the sun god. They had three daughters, namely Al-Lat, Al-Uzza, and Manat. They too were worshipped.
3. The promotion of Allah: Muhammad, the professed prophet of Islam, changed all of this and did his best to erase the history. He passionately opposed idolatry and by military and physical force destroyed all the gods in the Kaaba except the moon god Allah. Him he elevated by decreeing that he was the only god. But Allah's history survives, and archaeological discoveries regularly expose bits of his mythical identity and history. Muhammad tried to give him a new "revisionist" history by asserting that he is the same God Abraham worshipped and served, but historical facts are stubborn and will not accommodate him.
Why Was Allah "Promoted"?
Allah wat not initially even the central or main Arabian god. He was only one of 360, notwithstanding he was a "high" god. But, Muhammad was from the Quraysh tribe, and Allah was the principle god of the Quraysh tribe! Moreover, he was the favorite god of the family into which Muhammad was born. His father's name, for example, was Abd-Allah, indicating the particular devotion his ancestors had to the moon god before Muhammad's birth. It is no surprise Muhammad saved and promoted him when he destroyed all the other gods in the Kaaba.
In a single line, Muhammad started out with a pantheon of pagan gods, exalted the moon god (Allah), and thus wound up with one God. But he could no more be confused with Jehovah by a knowledgeable person than Baal, Isis, Molech or any other false god.
Allah's Nature
In an effort to exalt Allah, Muslims end up making him transcendent, unknowable, remote, and impersonal. The idea that man is made in his likeness or image is unthinkable. He has no essential essence ascribed to him as does Jehovah, such as love. Nor is he limited by his nature, as is Jehovah who, by reason of His nature, cannot lie. The Quran does not refer to him or ascribe to him the personal attributes that are Jehovah's. This has given rise to the idea with some Muslims that Allah is not a person or personality at all; he is higher than that. To them, to call him a person would be blasphemy (see "The Islamic Invasion" by Morey, page 59).
Conclusion
In a single, emphatic word, Christians and Muslims do not have the same God. They are altogether different. They differ as to their history. They differ as to their nature. They differ as to their attributes. They are altogether and drastically different.
As differences become apparent in a discussion or study, the ultimate Muslim explanation is that the Bible view of God is corrupted. That particular issue will of necessity be addressed when we compare the Quran and the Bible. We must examine the evidence to determine who is or what is right.
But right here it is an undeniable and irreconcilable reality - Muslims have an entirely different view of God than Christians do. This is not without reason. Allah is not the same God Christians worship. The difference is not just one of name, that is, of a different name for the same God. He is an altogether different god.
- slightly edited - Gospel Truths, October 1998