That's an arguable point, but I'll give it to you. And no need to get tiffy, I'm just trying to get a firm definition of the terms behind the debate.

One of the big reason I ask, is that the very premise of the Trinity (as defined by modern "Christian" theologians) is itself of questionable validity. Therefore, if a believe in the Trinity doctrine is a pre-requisite to being considered "Christian" (again, by whom: I'm not sure), then that is to throw out the possibility of the Trinity doctrine being incorrect, and debatable.
Therefore, a religion may subscribe fully to the belief that Jesus (of the Bible) was The Christ/Messiah, in all the capacity that the Gospels (as well as the rest of the Bible) describe, fulfill all the tenets set out by the New Testament, and yet still not be considered "Christian" simply because they may not accept the notion of the Trinity: (specifically the singular essence of God the Father, Jesus and the Holy Ghost).
Maybe its just me, but that is a bit ludicrous. An apt analogy (in my mind) would be to announce that a car with no windshield is no longer a car because all cars MUST have windshields.
Perhaps I am wrong?