I've worked in the field of cult apologetics for many, many (too many!) years. I personally believe that the most dangerous cult on the scene today is the Church of Jesus Christ of Latter-day Saints, otherwise known as the Mormons.
Joseph Smith, the church's first prophet, said that he was visited as a boy but God the Father and His Son, Jesus Christ, who told him that he was to "restore" the Christian Church because it was totally apostate, that all its creeds were an abomination, and that although Christians drew near God with their lips, their hearts were far from him. This took place sometime in the early 1800's, but it's hard to pin down because Smith gave us several versions of this "First Vision" Experience.
Smith was a known occultist, who dug up treasures in the earth by using a seer stone. He was convicted of "glass looking" in Chenango County, NY, after his "First Vision" Experience.
Supposedly an angel named Moroni led Smith to a hill in Palmyra, NY, where he located a box in the earth which had golden plates, upon which were supposedly enscribed "the fulness of the everlasting gospel" as written by the ancient inhabitants of North America.
According to the BOM, the Americas had been peopled by immigrants from the Holy Land in Old Testament times. Jesus appeared to them and set up his church in the Americas. The Nephites were the good guys who followed Jesus - the Lamanites, on the other hand, were the bad guys from whom the American Indians have descended. After numerous battles, the Lamanites wiped out the Nephites, except for one who hid the Book of Mormon plates on the Hill Palmyra in NY, or so the story goes!
In actuality, there is no doubt that Smith stole the basics of his BOM story from others, most notably, Ethan Smith, who wrote a book called "View of the Hebrews," a book contemporary with the BOM. The BOM deals with all the 19th century Christian controversies, and quotes major portions of the New and Old Testaments.
Although the BOM is the book used by Mormons to proselytize people into their cult - their real doctrines are contained in their other two books, Doctrines and Covenants and Pearl of Great Prince. To summarize their doctrines:
1. God was once a man, and through obedience to the laws of the gospel, evolved to godhood. God, Elohim the Father, created this world, and with one of his wives, had spirit children sent to the earth to inhabit human bodies.
2. The first one sent here was Adam, with his wife Eve. They sinned so that they could partake of "mortality" and have children - bodies for Elohim's children to inhabit.
3. The Christian church apostasized and was restored by Smith. Smith, and his successors (Mormon prophets) supposedly lead the church by revelation.
4. Good Mormons, who are married and "sealed" in secret temple ceremonies, and remain faithful to the Mormon gospel, can obtain "exaltation," which means they, too, can evolve into gods, create their own worlds, and people these worlds with their own spirit offspring. So, celestial sex is a key componet of Mormon salvation.
5. There are millions of these evolved gods in the universes, each with their own earths, etc. However, Mormons only are concerned with the god of this world, whom they call Elohim.
6. Mormons do not believe in the Trinity, but in three separate gods for this world: Elohim the Father, Jesus Christ (his first spirit child), and the Holy Ghost.
7. Mormons believe that Elohim had a sexual relationship with Mary in order to produce the physical body for Jesus. They deny that Jesus was begotten by the Holy Ghost.
8. Since Mormons believe that Elohim is a evolved man, they believe that all gods have glorified human bodies.
9. Salvation comes to all men, because Mormons teach that the resurrection of the body is salvation. However, "eternal lives" only comes to those on their way to exaltation to godhood," (obedient Mormons who have gone through the temple ceremonies).
Mormons love to appear as being Christian, but are in essence a group so far removed from traditional, orthodox Christianity, as to be the antithesis of Christianity.
In the US growth of the cult has slowed, but as the Mormons have spread out to countries where the Gospel of Jesus Christ is less known (especially South America), there has been considerable growth of the cult overseas.
I'd be happy to answer any more questions about this dangerous cult, should anybody have more questions.