Why Do Mormons Baptize The Dead?

Maybe you have seen the commercials for Ancestry.com. It is the largest for-profit genealogy company in the world. As of December 2013, it held records on 12.7 billion people. It is headquartered in Provo, Utah and was begun by two Mormon graduates of Bringham Young University. Why are the Mormons interested in your dead relatives?
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Since the Mormons believe baptism by immersion in the Mormon church is necessary for salvation, they compile elaborate genealogies of their “Gentile” ancestors and baptize them by proxy. Millions of the dead have been “baptized” and assured of salvation.