When they’re so clearly not. Just because you believe in Jesus doesn’t make you a Christian. Muslims, for example, see Jesus as a prophet, Baha’i’s see Jesus as a manifestation of God, yet clearly they’re not Christians.
All Christians profess the Trinity and the incarnation of Jesus Christ as the second person of the Trinity. This is basic Christian doctrine that transcends denominational lines, even from the most Traditional Latin Catholic to the most liberal kind of Lutherans, even from the most mystical Orthodox practitioner to the most Fundamentalist KJV only fire and brimstone Independent Baptists; the Trinity is held to be central within each of these Christian groups no matter how much they radically differ from each other on other doctrinal issues.
To put it simply, the Trinity is the unanimous doctrine of Christianity. You cannot be a Christian without the Trinity. Christ himself affirms this when he tell us to baptize in the name of the Trinity (Matthew 28:19). From the outside, it appears they do the same, but when it’s shown what they actually believe in, it all becomes void. Mormons profess three gods instead of one. They say that the God has flesh and bone, that he has his own goddess wife, that he lives on a planet/star named Kolob. To most Christians, these are the worst kinds of blasphemies. It sounds like paganism! In fact, it is pagan!
How do they have the audacity to claim they’re Christians when they reject the most fundamental Christian truths? How can they claim to be a follower of Christ yet admit to the worship/service of 3 gods!? Did they miss it when Christ said, “No man can serve two masters” (Matthew 6:24)? Yet they claim three! Mormonism is blasphemy against God. It is incompatible with Christianity. You are either a Mormon, or you are a Christian. Pick one. Being both is impossible.
Out of love, I pray for Mormons, that they will discover Christ and come to the truth.