Mohammed can legitimately be called Islam’s prophet but no Christian can with integrity refer to him as ‘The Prophet’.
The reason is very clear. Christians believe that the divine Being at the heart of their faith, the Lord Jesus Christ, is God Incarnate and that He is the Second Person of the Trinity. The religious book attributed to Mohammed, the Koran, explicitly denies both the Incarnation and the Trinity. The two convictions cannot both be right.
Christians who say and mean the historic creed authorised by the Catholic Church in the 4th Century AD, the Nicene Creed, necessarily believe that the Koran, which emerged around three hundred years later, is wrong about the nature of the true God:
“I believe in one God the Father Almighty, Maker of heaven and earth, And of all things visible and invisible: and in one Lord Jesus Christ, the only-begotten Son of God, begotten of his Father before all worlds, God of God, Light of light, Very God of very God, Begotten, not made, Being of one substance with the Father, By whom all things were made: Who for us men and for our salvation came down from heaven, And was incarnate by the Holy Ghost of the Virgin Mary, and was made man” (from the Nicene Creed as set out in the Book of Common Prayer’s Order for Holy Communion).
Muslims, both moderate and militant, do not believe what the Nicene Creed proclaims about God.
If you are an Anglican, then you accept the Nicene Creed along with the Athanasian and Apostles’ Creeds because, as Article 8 of the Church of England’s 39 Articles of Religion clearly states, “they may be proved by most certain warrants of holy Scripture”.
Given that Mohammed was so clearly wrong about the Trinitarian nature of the true God and about the Incarnation, then he is not ‘The Prophet’. He was not the man through whom the true God chose to speak authoritatively and definitively.
Furthermore for the Nicene Christian, Mohammed was not even a prophet of God. That is because, biblically, a true prophet was a man or in a minority of cases a woman such as Deborah or Huldah to whom the Lord God Almighty chose to reveal His authoritative spiritual and moral truth for his chosen people at a particular time.
For example, Christians believe that God chose to reveal His truth to the Old Testament prophet Isaiah – he of “the people that walked in darkness have seen a great light” – albeit not all the truth He wanted eventually to reveal. It was the Incarnate Son of God, to whom Isaiah pointed, who revealed the consummate divine truth which the Old Testament prophets foreshadowed.
That does not mean Mohammed, as a man made in the image of God, was not capable of conveying valid spiritual and moral insights. The Koran does communicate some true insights and some good morals. But the Nicene affirmations do mean he was not ‘The Prophet’ or even a divinely-appointed prophet in the biblical sense.
No orthodox, Nicene Christian need gratuitously offend Muslims by depicting Mohammed but he or she necessarily should risk offending them by refusing to refer to him as ‘The Prophet’.