Mormonism is one of several dozen different religions that started up around the same time during a period of American History referred to by some as “The Great Awakening” a period of religious fervor in which they teach that lost truths of the early Church were re-discovered, and restored by or through their own perticular prophet or founder. These groups of Restorationists include the SDA, Jehovah’s Witness’, and the Church of Christ, and several other groups who share similar concepts.
First most denied the Divinity of Christ, the existance of the Trinity, and they believed that the “True” Church had long since disappeared from the face of the earth, (Despite what Jesus promised us.) They all have a very anti-Catholic bent to them. If one holds that their teachings are the true teachings lost for centuries because the Catholic Church or some other group conspired to take power, it is easy to dismiss historical and archiological facts, and create your own counter history. Mormons and others have been taught a history different from what most reputable historians hold, and are entrusted with what they think is the sacred obligation to hold the rediscovered truth.
As I mentioned, above Mormons deny Jesus is the Second person of the Trinity. They believe that the God of this world was once a man, who has progressed to becoming a god and has populated the earth by having sexual relations with his wives. They believe that they are blood brothers of Jesus, and one of his brothers was Satan, who was not evil per-se, but upset that in their “pre-earth” life Jesus was picked over him to be the savior. So they don’t really believe in a devil who is eternal, and surmise that Satan will one day become a god himself. I’ve heard the story that Joseph Smith was shown the plates on which the BOM was written by the “Angel Moroni” There was also a document in early Mormon History called the Salamander Papers, which the Church of Mormon once accepted, but after they were proven as a forged document, now denies. I’ve never heard that they were given by a serpant, but then again, there are so many things that Mormonism has changed, that it’s hard to keep up.