In response to the argument that Christians and Muslims worship the same God, D. A. Carson makes this point.
[W]hen Jesus is asked, “Show us the Father” (John 14:8), he replies, “Don”t you know me, Philip, even after I have been among you such a long time? Anyone who has seen me has seen the Father. How can you say, ‘Show us the Father’?” (14:9) It appears, then, at this point in redemptive history, that not to recognize who Jesus is, is not to know God.”1
In the verses quoted from John 14, Jesus is asserting his own deity. If those who have seen Jesus have seen the Father, then he is God just like the Father is God. Now, after his coming, those who do not recognize Jesus as God do not actually know or worship the God he came to reveal.
Christians and Muslims may both claim to worship the God of Abraham, but anyone who doesn’t acknowledge the deity of Jesus is actually worshipping another god, and not the same God Christians worship.
1] D. A. Carson, The Intolerance of Tolerance