The person that the mormons believe in is not the same Jesus in the bible. Christians believe facts about Jesus Christ that are necessary to be a Christian. The mormons do not believe those truths. For example the mormons believe that Jesus is the brother of Satan. They believe that he is an angel created by the FAther. They believe many things that contradict the Christian bible. They believe different things about the resurection and many things that are in the book of mormon are no where in the bible. They do not believe that Jesus is God. They do not believe in the Trinity. They believe that God was a man before he became God. There are othere bizzare things that you will have to read about their teachings to see how unchristian they are.
Been there, done that. BIC 1941.
Ed Decker, former Mormon, outlines all the beliefs that are contrary to Christianity on his website.