The Book of Mormon is a record of ancient scripture written on the American continent and preserved in upstate New York until it was revealed by an angel in the 1800s. The people who kept the record were descendants of several families who immigrated from Jerusalem near the fall of that city. The Book of Mormon does not mention currently existing civilizations, but most Mormons do not currently believe the continent was empty when the people of the Book of Mormon arrived. The book, which was condensed by the last two writers of it, simply doesn’t mention others.
The prophets of the two civilizations that developed from these people prophesied concerning what would someday be the United States of America. In Second Nephi, the prophet Lehi recorded a vision concerning the continent and the future nation. He was told the land was covenanted to his family and to others who would come to it from around the world by the hand of God. He learned that no one would come to the land unless brought by the hand of God.
Lehi was told in the vision that if the people who lived there would keep the commandments, the land would be one of liberty and would never be taken into captivity by another nation.
He learned that God intended the land to be kept secret from the world for a time, to keep it from being overrun before the time came for it to be discovered. As long as the people God sent from Jerusalem were righteous, the land would be theirs to enjoy, but if they became wicked or stopped believing in God, outside nations would be permitted into the land and persecution would result. (See 2 Nephi 1 .)
Lehi told his son Joseph that another Joseph would one day be born who would be the first prophet in the restoration of the gospel in the last days. This prophecy refers to Joseph Smith.
Lehi’s fourth son, Nephi, also had a vision of America. He saw a man whom we know today to be Christopher Columbus setting out and finding America, as did others. He saw the persecution of the Native Americans, including whatever few remnants of his own family might remain. (Many were destroyed during a great war at the end of the two civilizations.) However, he was promised they would not be destroyed completely.
Lehi saw the United States being formed and the coming forth of the Book of Mormon, the record Lehi would begin. It would continue to be handed down through the generations, with various religious and government leaders recording the history of the people and their religious teachings and experiences. The pivotal event in the Book of Mormon is the appearance of Jesus Christ on the American continent. Sometime after His death and resurrection, He would appear to them for several days, teaching and blessing them. He let the world know, with this visit, that He was not just the Savior of the Jews, but of the entire world. Because He came to them after His death and because they knew who He was through continued prophecies, it proves the divinity of the Lord Jesus Christ. The Book of Mormon testifies of Jesus Christ and of the Bible. It is, in fact, impossible to believe the Book of Mormon without also believing in the Bible.