On reason of the interreligious dialogue - and maybe also on the reason to show, that he was not full of mistrust against all Muslims only because a Muslim once tried to murder him. And as far as I understood Mohammed said only that people who believe in the trinity "God father, mother Mary and Jesus" are not Christians. And Mohammed was not wrong to say so. The holy family is Joseph, mother Mary and Jesus - the tri-unity is God father, the Holy Spirit and Jesus, the son of god.
In general: Muslims don't share our belief in the triune god - nevertheless they and we believe in the same god, because there is only one god. The concept "tri-une" is not so difficult to understand as lots of people think. The space for example has different directions: fortward-backward, left-right, up-down. But there's always only space in all directions and everywhere are this three dimensions. Without no one could find any way. And please: I said not god is space - I showed only that it is not difficult to understand the word tri-une.