If the LDS are right about who God is, then God and His word are wrong.
In the Bible we have doctrines that are expressed by Jesus Himself, The one and only God incarnate, none ever before Him and none after Him anywhere or anytime. These doctrines are repeated and expounded upon by the Apostles in the Epistles of the New Covenant recorded in the New Testament. These doctrines are exclusive claims to actual truths. When someone else confesses a doctrine that asserts something other than what has already been established by the one true, immutable, and infallible God, then our only option is to reject their assertion as heresy.
Let’s look at some of their doctrines that conflict with Biblical doctrines, and are mutually exclusive in their claims. Christians claim there is only one true God. There has never been any other god besides Him. He has always existed in trinity. He has never not existed. We also claim that if you don’t believe in Him as explained here, as one being in three personages, that you can not be called part of the Church. Because, you have put your faith in a different god. Obviously, worshipping a different god makes you not a Christian.
Latter Day Saints (LDS or Mormons) do not have one triune God. They actually have a number of gods approaching infinity. They choose to worship just one of the gods Joseph Smith imagined for them. The god they choose to worship, was once a man named Elohim. He lived on a planet orbiting a star called Kolab. So their god is an E.T. extra terrestrial, alien from outer space. Allegedly he was such a good follower of the mormon doctrines that he was promoted to being an exalted man. He eventually became the god of this solar system and the Earth that we live on. So the one god that they worship out of the billions of gods they have, was once a created man. He was born of his father god and goddess mother. They now call him Heavenly Father and believe he has a body of flesh and bone. They believe that he had sex with Mary the virgin and that Jesus the spirit child was then embodied in his biological son with Mary. Here is a quote from the Brigham Young University site,
As the Prophet Joseph Smith said, “God himself was once as we are now, and is an exalted man, and sits enthroned in yonder heavens! That is the great secret” (TPJS, p. 345). Thus, the Father became the Father at some time before “the beginning” as humans know it, by experiencing a mortality similar to that experienced on earth. There has been speculation among some Latter-day Saints on the implications of this doctrine, but nothing has been revealed to the Church about conditions before the “beginning” as mortals know it. The important points of the doctrine for Latter-day Saints are that Gods and humans are the same species of being, but at different stages of development in a divine continuum, and that the heavenly Father and Mother are the heavenly pattern, model, and example of what mortals can become through obedience to the gospel (see Mother in Heaven). Knowing that they are the literal offspring of Heavenly Parents and that they can become like those parents through the gospel of Jesus Christ is a wellspring of religious motivation. With God as the literal Father and with humans having the capacity to become like him, the basic religious questions “Where did I come from?,” “Why am I here?,” and What is my destiny?” are fundamentally answered.
Keep in mind that our LDS friends co-opt many of our words. They mean something completely different to them. When we talk of God the Father or Jesus we are talking about a very different God than they are. They might believe we are talking about the same god, but they have better and more true information than we do. The gospel to them is not the same things we think of. Their gospel is a bunch of laws, rules, do’s and don’ts, from their book Doctrine and Covenants. Our understanding comes from the Biblical definition. The word, “gospel” means, “good message or news.” When we think of the gospel of Jesus, we understand it to mean the penal substitutionary atonement wrought by Christ God Himself for the salvation of His sovereignly elected saints to bring glory to His name. In Christianity the work has been done by Jesus. He paid for our sins. By repenting and believing in Him, his work is imputed to us. It is done, done, done. We do good works because of our new affections and desires born from a regenerated spirit as born again believers.
with this article on only of few of the many differences, I hope it is obvious that the LDS are not Christian. They are a polytheistic Arian cult, created by the false prophet Joseph Smith.