Do Christians and Muslims worship the same God? The answer is an unequivocal and unambiguous “No.” Muslims themselves will confirm this to you if you know the questions to ask.
We can stipulate that “Allah” is the generic word for “God” in Arabic, just as “El” and “Elohim” are in Hebrew and “theos” is in Greek. Thus there are many Christians in the Arabic speaking world who refer to the God of the Bible as “Allah,” and who use it in phrases such as “Inshallah,” which means “God willing.”
(It’s worth noting in passing that the highest court in Malaysia recently ruled that Muslims and Muslims alone are permitted to use the word “Allah.”)
But generic words for God, because they are generic, can be used to refer to a multiplicity of gods. So the term must be narrowed down. If someone uses a generic word for “God,” the follow-up question must be asked, “Which ‘god’ are your referring to?”
In New Testament times, where Greek was the lingua franca of the civilized world, there was a virtually unlimited pantheon of both Greek and Roman “gods,” all of whom were identified using the generic term “theos.” (As you might guess, we get our word “theology” from this word.)
So when the apostles needed to make sure their readers knew which God they were talking about when they used the word “theos,” and that they were referring to none of the Roman and Greek gods, they added a clear qualifier. They referred to the true and living God as “the God and Father of our Lord Jesus Christ” (Ephesians 1:3; 2 Corinthians 1:3; Colossians 1:3; 2 Peter 1:3).
It was their way of saying, “Look, the God I am referring to here is not Zeus, or Jupiter, or Hermes, or Mercury, or Neptune, or Diana or Aphrodite. The God I am talking about is the God who is the Father of our Lord Jesus Christ.”
Jesus, according to the New Testament, is the unique, one and only begotten Son of God. (We can become sons of God by adoption, but he is the Son of God by his very nature.)
Here is the point. The God of the Bible has a Son. The god of Islam does not.
In fact, Muslims in 2008 hung a large banner in front of the Basilica of the Annunciation in Nazareth stating flatly that Allah has no son, and quoting a passage from the Qur’an as proof: “He begetteth not, nor is begotten, and there is none like unto him” (Surah 112:1-4).
Contrast this with this declaration from the gospel of John: “And the Word became flesh, and dwelt among us, and we saw His glory, glory as of the only begotten from the Father, full of grace and truth” (John 1:14, NASB).
And again we read in John 3:16, "For God so loved the world, that He gave His only begotten Son, that whoever believes in Him shall not perish, but have eternal life.”
The plain declaration of Christianity, then, is that Jesus is the eternally begotten Son of the true and living God. There never was a time, not even in eternity past, when he was not the Son of God.
But Islam, on the other hand, flatly denies that Allah has a son at at all. “He begetteth not, nor is begotten.” In fact, believing that God has a begotten Son will get you stoned to death in many parts of the Muslim world.
On top of all this, and of particular offense to orthodox Muslims, is the fact that Christians worship Jesus himself as God as the second member of the Trinity. “In the beginning was the Word, and the Word was with God, and the Word was God” (John 1:3). Muslims are horrified at the thought that Jesus could be worshipped as God and consider such a belief as blasphemy worthy of death.
So, do Christians and Muslims worship the same God? Absolutely and unequivocally not. If you doubt me, ask a Muslim.