Until Gordon B. Hinckley became president of the Mormon church in 1995, Mormons focused almost exclusively on what makes them different, unique, and preferable to what we might call historic Christianity in any and all of its forms (i.e., Catholicism, Orthodoxy, or all strands and strains of Protestantism). At root, this focus revolves around some very foundational Mormon claims, particularly what they call "The Great Apostasy." According to Mormons, sometime shortly after the end of the apostolic age, the Church founded by Christ and His apostles deviated from true Christianity. These deviations, over time, continued to grow deeper and deeper, as well as to multiply in number.
Of course the Mormon belief in The Great Apostasy presumes almost utter ignorance of the early Church Fathers, especially the so-called Apostolic Fathers, men such as Polycarp of Smyrna, Ignatius of Antioch, Clement of Rome, Papias of Hieropolis, not to mention documents like the Didaché and the Shepherd of Hermas, as well as Ireneaus of Lyons, Justin Martyr, etc. It also requires a fairly superficial reading of several of St Paul's letters. While, according to this view, there were attempts here and there, such as the Protestant reformation, to restore authentic Christianity, the true Church established by Christ and the Apostles was absent from the earth.
From the Mormon perspective it was Joseph Smith Jr. who restored the true Church after it having been absent from the earth for the vast majority of time between Christ's coming and the nineteenth century. For the past twenty years, however, Mormons have focused more on insisting that they are Christians, like Catholics, Orthodox, Lutherans, Anglicans, Methodists, Presbyterians, Baptists, etc. So it is refreshing these days when someone in leadership in the Mormon Church, someone who is what they call a "General Authority" (i.e, a member of the First Presidency, Quorum of the Twelve Apostles, First and Second Quorums of the Seventy, or the Presiding Bishopric) sets about to tell the truth about Mormon distinctiveness vis-à-vis historic Christianity. So I appreciated very much reading a brief article in the Mormon Church-owned Deseret News about a presentation given earlier this month by Bishop Gérald Caussé, who serves as Second Counselor in the Presiding Bishopric, to new Mormon mission presidents and visitor center directors.
The bottom-line-up-front comes from the end of the article: "Investigators [those considering becoming Mormons and being taught by proseltyzing missionaries] must understand that a universal apostasy occurred after the deaths of Christ and His apostles; otherwise, there would have been no need for a Restoration." This dovetails nicely with the beginning of the article, which cites Caussé as insisting that "The Church of Jesus Christ of Latter-day Saints is not just another Christian Church, but rather it has unique elements that are essential to the salvation of mankind." In between Caussé is reported as teaching that there are "three essential truths conveyed in three major events associated with the Restoration." First, "the vision of the Father and the Son experienced by the Prophet Joseph Smith in the spring of 1820" (see "Latter-day quandry and resolution"). Second, "the translation of the Book of Mormon." Third, "the restoration of priesthood keys and ordinances by heavenly messengers."
I don't post this to be contentious or even really to argue, but simply to demonstrate, prior to any subsequent discussion, the objective differences between Mormonism and Christianity as handed on and practiced throughout its history. I think it's terribly important to be honest and forthright about our objective differences. Such honesty is the only way to have meaningful interaction. There certainly are points of contention and disagreement among Christians. But when compared to our collective differences with Mormonism, these differences (i.e., the nature and structure of the Church, authority of the Church, sources of revelation- Scripture only, or Scripture and Tradition?- sacraments, justification, the relationship between faith and works, etc.) are quite narrow.
Among Christians Catholic, Orthodox, and Protestant there is no dispute about the triune nature of God, who is Father, Son, and Holy Spirit, or that the Holy Bible is what constitutes Scripture (even if some accept the Deutero-canonical books as inspired and some do not), or even that the Church of Christ was given irrevocable authority ("the gates of the netherworld [/hell] shall not prevail against it" Matthew 16:18) to act in the name of Christ, especially as it pertains to baptism, the Lord's supper, and handing on the truth of the Gospel and has existed on the earth since its establishment.
Once again it bears noting that the question, "Are Mormons Christians" is the reddest of herrings. By their own account, Mormons are the only true Christians. This is borne out by the fact that everyone who becomes Mormon must be baptized, even if they were previously baptized "in the name of the Father, and of the Son, and of the Spirit," even if s/he was baptized by immersion after having reached the age of reason. Besides in the official account of Joseph Smith Jr's "First Vision," in which God the Father appeared to him in bodily form, thus revealing that even God the Father "has a body of flesh and bones as tangible as man's" (Doctrine & Covenants 130:22), the Father says these words to Smith in answer to Smith's question about which church he should join: "I was answered that I must join none of them, for they were all wrong; and the Personage who addressed me said that all their creeds were an abomination in his sight; that those professors were all corrupt; that: 'they draw near to me with their lips, but their hearts are far from me, they teach for doctrines the commandments of men, having a form of godliness, but they deny the power thereof'" (Joseph Smith History v 19).