Are Mormons "Christians" as defined by traditional Christian orthodoxy? The answer to that question is easy and straightforward, and it is "no." Nevertheless, even as the question is clear, the answer requires some explanation.
We are not talking here about the postmodern conception of Christianity that minimizes truth. We are not talking about Christianity as a mood or as a sociological movement. We are not talking about liberal Christianity that minimizes doctrine, nor about sectarian Christianity which defines the faith in terms of eccentric doctrines. We are talking about historic, traditional, Christian orthodoxy.
Once that is made clear, the answer is inevitable. Furthermore, the answer is made easy, not only by the structure of Christian orthodoxy (a structure Mormonism denies), but by the central argument of Mormonism itself - that the true faith was restored through Joseph Smith in the nineteenth century in America and that the entire structure of Christian orthodoxy as affirmed by the post-apostolic church is corrupt and false.
In other words, Mormonism rejects traditional Christian orthodoxy at the onset - this rejection is the very logic of Mormonism's existence. A contemporary observer of Mormon public relations is not going to hear this logic presented directly, but it is the very logic and message of the Book of Mormon and the structure of Mormon thought. Mormonism rejects Christian orthodoxy as the very argument for its own existence, and it clearly identifies historic Christianity as a false faith.
So, what does Mormonism reject? The orthodox consensus of the Christian church is defined in terms of its historic creeds and doctrinal affirmations. Two great doctrines stand as the central substance of that consensus. Throughout the centuries, the doctrines concerning the Trinity and the nature of Christ have constituted that foundation, and the church has used these definitional doctrines as the standard for identifying true Christianity.
Contemporary Mormonism presents the Book of Mormon as "another testament of Jesus Christ," but the Jesus of the Book of Mormon is not the only begotten Son of God, the second person of the Trinity, or the one through whose death on the cross we can be saved from our sins.
Taken from "Mormonism Is Not Christianity" (used by permission).