Mormons use Christian terminology, but have changed all the definitions to mean something completely different from what Christians mean by them. Let's compare orthodox Christianity, such as it has existed for centuries and is proclaimed by evangelicals today with Mormonism.
Let's take the person of God, for example:
Christians believe that God exists as a Trinity, that is, one God existing in three person. Mormons believe that there are not just three separate gods, but that there are multiple gods governing other planets (Mormon Doctrine, p. 163). Therefore, they are polytheists, while Christians are monotheists.
Christians believe that the Triune God has always existed as God and God is Spirit. Mormons believe that God was once a man who became a god ((Mormon Doctrine, p. 321; Joseph Smith, Times and Seasons, vol. 5, p. 613-614; Orson Pratt, Journal of Discourses, vol. 2, p. 345; Brigham Young, Journal of Discourses, vol. 7, p. 333).
Smith, in his teachings, mocked the Trinity, saying that the idea of three gods "crammed into one" would make "a wonderfully big God -- he would be a giant or a monster" (p. 372).
Then there's the identity of Jesus:
Christians believe he is the second person of the Triune God and, as such, as always existed as deity. Mormons believe he was born as a spirit (Mormon Doctrine, p. 29).
Christians believe that Satan was a fallen angel. Mormons believe Jesus and Satan are spirit brothers and we were all born as siblings in heaven to them both (Mormon Doctrine, p. 163; Gospel Through the Ages, p. 15).
Christians believe Jesus was born of a virgin. Mormons believe God had sexual relations with Mary to give Jesus' spirit a body (Brigham Young, Journal of Discourses, vol. 4, 1857, p. 218; vol. 8, p. 115 and Mormon Doctrine by Bruce McConkie, p. 547).
Then there's the issue of salvation:
Christians believe that we are saved by faith in Christ and salvation is a gift (Roman Catholics add works, but that is contrary to what the Bible says). Mormons disagree. As Spencer W. Kimball wrote in Miracle of Forgiveness (p. 206), "One of the most fallacious doctrines originated by Satan and propounded by man is that man is saved alone by the grace of God; that belief in Jesus Christ alone is all that is needed for salvation."
In the Journal of Discourses, vol. 3, 1856, p. 247, we read that Jesus' sacrifice was not able to cleanse us from all our sins, (murder and repeated adultery being the exception).
The Mormon Doctrines of Salvation, vol. 1 tells us that there is no salvation without accepting Joseph Smith as a prophet of God (188).
While Mormons will tell you that they accept Christ's atonement, the reality is that Christ only atones for their sins if they repent of them, are baptized in the Mormon church and obey all the commandments" (Gospel Principles, Corporation of the President of the Church of Jesus Christ of Latter-day Saints, 1979, p. 68.). In other words, their "salvation" is works-oriented. It is not a gift of grace as Christians teach and believe.
Mormons have traditionally taught that if a man is a good enough Mormon, he can become a god and have his own world to rule. This is something that the Mormon church is currently trying to play down. But it was stated in the Teachings of the Prophet Joseph Smith, p. 345-347, 354, and was taught right up until this century.
When Christians speak of becoming like Christ, we are talking about taking on his character, not earning divinity.
Then there is how we regard the Bible.
Christians believe it is the Word of God. Mormons believe it is the Word of God except where it differs from Mormon teaching. Then, they say, it is corrupt. Christians practise exegesis, letting the Bible speak to them. Mormons practise eisegesis, twisting the Bible to make it fit their preconceived beliefs.
Then there's the Book of Mormon which has NO archaeological data to back it up. The Bible is backed up by a great many historical discoveries and facts.
Then there's the personage of Smith himself who made all kinds of prophecies that didn't come true. The Bible says that, when a person's prophecies do not come true, then he is not a true prophet of God. The only prophecies in the Bible that have yet to come true are those with regard to Christ's second coming -- when they will all be fulfilled.
Not all Mormons will believe all of the things I listed above. There are different branches that believe different aspects of Mormon theology. But none of them line up with Christianity.
One of the problems with Mormon theology is the fact that it is fluid. Whoever heads the church has the right to make changes about anything and everything, supposedly according to what God tells him.
For example, at one time, polygamy was a requirement for godhood so men had to "marry" many women. Because of societal pressure, Mormons decided to drop that as a prerequisite. Polygamy was instituted as a requirement because Smith was having sexual relationships with more than one woman and wanted to legitimize it. While there is polygamy in the Old Testament times, it was NEVER commanded or condoned by God.
At one time, blacks were said to be of Satan. The civil rights movement led them to stop teaching that claim. The Bible tells us that all are one in Christ no matter what their race.
I have offered only a very few of the literally dozens differences. I haven't even touched the issue of heaven and the three levels that Mormons teach about.
In fact, there are far too many differences to list here. The interesting thing is that Joseph Smith considered Christianity to be false and hated the very word "Christian" and didn't want to be known by it. But in the last few decades, Mormons have decided that they very much want to be seen as Christians, perhaps because they thought they would gain more acceptance by being in the Christian camp.
But the LDS church is no more Christian than Islam or Hinduism are. Mormonism is a completely separate religion. As I said, Mormons have borrowed Christian terminology, but they mean different things by them. That's why people who know nothing about the LDS are misled by them. The LDS church is attempting to hijack Christianity. They can fool non-Christians who don't know any better, but they can't fool Christians.
Does that make all Mormons horrible people? Not at all. There are some very nice Mormons in the world. Unfortunately, they are being deceived by their own church. I have no problem with Mormons saying they are Mormons, but it does bother me when they say they're Christians because, whether they know it or not, it just isn't true.
Bill McKeever keeps on top of all the latest Mormon trends and news. I recommend his site:
http://www.mrm.org/
I spent a year and a half dialoguing with a Mormon man about all of these things. So, while I have researched the information (for a major paper in a World Religions class), using books, web sites, etc., I have had what I read confirmed by a devout Mormon. There may be Mormons who disagree with him, but he didn't deny any of my claims -- although he did try to sidestep a few issues along the way!