It seems clear that someone whom two neighbours know as "Jones", who lives in the house on the hill, is the same person, even if each of them knows different things about him, or knows him differently. Each has a drawer full of letters from
him, or perhaps letters about him from different people, which seem to paint him in different colours. But one of the neighbours also claims to be a close relation of Jones, and to know him as family.
Christians claim to know more of God than Muslims know, because of what has been revealed to them in faith (the Incarnation, the Trinity). Of course, to know God as Trinity will not do them much good unless they also become holy and receive the Holy Spirit into their souls, allowing themselves to be transformed by his presence. To know certain things about God is better than knowing nothing, or knowing something false, but it is not the same as knowing God.
In thinking about the differences between the Christian and Muslim understandings of God, it is sometimes helpful to consider the fact that the Jews, too, believe in a God who is One, not Three, and reject the idea that he had a Son who died on the Cross for us. Yet most of the Christians who deny the identity of God and Allah would accept that the Jews worship the same God as they do, albeit in ignorance of certain important facts about him that were revealed later.
That is because Christians interpret the Jewish Scriptures as pointing towards the revelation of Christ, taking a lenient view of those Jews who cannot see this, or who cling faithfully to the Scriptures they have been given. Whereas, of course, the Quran comes after Christ and appears to be pointing away from him. Jewish Scriptures are therefore taken as inspired and true – even the bloodthirsty psalms, and the mass slaughter perpetrated by the Jews at God's behest, which come to be interpreted allegorically or spiritually. Muslim Scriptures, on the other hand, are rejected as deceptive and misleading.
Yet what if the denials of Trinity and Incarnation and so forth in the Quran are not as unambiguous as we have been led to believe? And it is the case that several of the supposedly Christian doctrines rejected by Islam, in the form that they are rejected, are not Christian at all, but heretical: the idea that God can beget a Son physically, for example, or the idea of a divine Trinity comprising God, Mary, and Jesus.
Should we not say that, while the two traditions are speaking of the same God, they believe different things of him, and (to the extent these things are contradictory) they cannot both be right? The important questions then are to do with the degree of actual contradiction, the reasons for it, and the consequences of it – including the behaviour of believers – rather than whether they are speaking of the same individual. (There are Orthodox who claim to worship a different God from the Catholics merely because they do not accept the filioque clause in the Creed. That seems to me perverse.)
Peter D. Beaulieu, Beyond Secularism and Jihad: A Triangular Enquiry into the Mosque, the Manger, and Modernity (University Press of America)
The Tabah Foundation
A Common Word
Interview with Remi Brague