OK, this will take awhile.
Mormons believe the Bible as Christians know it is full of errors. They, as an example, believe that The Father, The Son and The Holy Spirit are 3 distinct beings, not a Triune God, as all Christians do.
They believe that God was once an ordinary man, who through Faith, Clean Living and Hard Work, became an exalted Being. This is Sacrilege of the highest order to any Christian.
They believe Jesus and Satan are literal brothers, and that Jesus's version of Salvation was better, and God Chose His.
Christians believe Salvation is God's Plan.
They believe God changes his mind, and offers new revelations to the chief prophet of the church. He did this when polygamy was, then wasn't a part of their belief system. Also when African Americans weren't, then were eligible for inclusion into the Sainthood. The Christian Bible tells us not one 'word' shall be added or taken from His Revealed Word. To be a Mormon, you must believe God lied when he inspired this writing
Mormon's believe we are born of a celestial union between their ManGod, and his exalted 'wife'.
That is why Mormon's marriages are private, they are not being married, but sealed for eternity and given secret names so they will be able to know each other in heaven.
They do not believe The Bible is the wholly Perfect, Inspired and Inerrant Word of God.
They do believe that Jesus is Their Savior, and on that sole definition, they could be called Christians. But since their God is different than the Christian God, their Jesus is different than The Christian Jesus, Their Holy Spirit different than the Christian, their Satan is different than the Christian Satan, their Scripture is Sacred, the Christian's is full of errors and their Creation of Life is different than the Christians, and their god can change his mind, how can they be Christian?
The essential elements of Christianity were long and well known long before Joseph Smith recieved his revelation. There are hundreds of writings, essays, studies and histories written by the early Saints on specifically that they are not to associate with Christians, that they are heathens. A 15 minute search on the internet, and you could turn up a few of your own. It is only the last 30 years or so this has change has occurred, as they desire to become more mainstream.
Now, I have no quarrel at all with what they believe, how they worship, or how they live. The Saints are not evil, they are not devil worshippers, they are not ready to take over the USA. They are regular, ordinary people living life as best they can, and I admire them. But they believe differently.
Why is this important?
Because anything that blurs the line between the Truth of God and His Word, and an almost look alike is harmful to Christianity. Christians have a heard enough time making distinctions between Christian denominations as it is. The more God's Word is diluted and/or blurred, the harder it will be for Believers to find HIm.
If you were a baseball player, and I said to you that I am a baseball player also, and I held a bat, a ball, and a mitt in front of you, you'd believe me. The same tools with the same names. But if I hiked the ball to you, slam dunked the mitt, and slapped shot the base into a net while ice skating, am I really a baseball player?
Now the Saints say they believe in Jesus as their Savior, and the He died for their sins. They also will say Christ is the center of their life.
And I believe them. And you believed me when I said I was a baseball player.
That is the problem with the Saints being called Christians.
Geez, I hope this helps. God Bless you all!