1) Joseph Smith taught that baptism for the dead was necessary for perfection.
Doctrines & Covenants 128:15-16 And now my dearly beloved brethren and sisters, let me assure you that these are principles in relation to the dead and the living that cannot be lightly passed over, as pertaining to our salvation. For their salvation is necessary and essential to our salvation, as Paul says concerning the fathers--that they without us cannot be made perfect -- neither can we without our dead be made perfect. And now, in relation to the baptism for the dead, I will give you another quotation of Paul, 1 Corinthians 15:29: Else what shall they do which are baptized for the dead, if the dead rise not at all? Why are they then baptized for the dead?
Correction: Baptism for the dead was a pagan practice in ancient Corinth.
1 Corinthians 15:29-30 Otherwise, what will they (pagans in Corinth) do who are baptized for the dead, if the dead do not rise at all? Why then are they (pagans in Corinth) baptized for the dead? And why do we (Christians in Corinth) stand in jeopardy every hour?
2) Joseph Smith taught that baptism for the dead was the final mystery.
Doctrines & Covenants 128:18 I might have rendered a plainer translation to this, but it is sufficiently plain to suit my purpose as it stands. It is sufficient to know, in this case, that the earth will be smitten with a curse unless there is a welding link of some kind or other between the fathers and the children, upon some subject or other--and behold what is that subject? It is the baptism for the dead. For we without them cannot be made perfect; neither can they without us be made perfect. Neither can they nor we be made perfect without those who have died in the gospel also; for it is necessary in the ushering in of the dispensation of the fulness of times, which dispensation is now beginning to usher in, that a whole and complete and perfect union, and welding together of dispensations, and keys, and powers, and glories should take place, and be revealed from the days of Adam even to the present time. And not only this, but those things which never have been revealed from the foundation of the world, but have been kept hid from the wise and prudent, shall be revealed unto babes and sucklings in this, the dispensation of the fulness of times.
Correction: The final mystery is that Gentiles can be saved through the grace of God.
Ephesians 3:1-7 For this reason I, Paul, the prisoner of Christ Jesus for you Gentiles; if indeed you have heard of the dispensation of the grace of God which was given to me for you, how that by revelation He made known to me the mystery (as I have briefly written already, by which, when you read, you may understand my knowledge in the mystery of Christ), which in other ages was not made known to the sons of men, as it has now been revealed by the Spirit to His holy apostles and prophets: that the Gentiles should be fellow heirs, of the same body, and partakers of His promise in Christ through the gospel, of which I became a minister according to the gift of the grace of God given to me by the effective working of His power.
1 Peter 1:20-21 He indeed was foreordained before the foundation of the world, but was manifest in these last times for you who through Him believe in God, who raised Him from the dead and gave Him glory, so that your faith and hope are in God.
3) Mormons believe that their ancestors can gain salvation through a proxy baptism .
All ordinances for the dead had to wait until after Jesus Christ had gained the victory over death, and then to Peter and his brethren the authority to officiate in behalf of the dead was revealed. Unfortunately, there is little written in the New Testament giving the history of such ordinances having been performed. Paul has given us the clearest insight in his first epistle to the Corinthian saints wherein he spoke of baptism for the dead, implying that this principle was very well understood. Answers to Gospel Questions, Volume 2, Joseph Fielding Smith.
Correction: There is no salvation after death.
Hebrews 9:27-28 And as it is appointed for men to die once, but after this the judgment, so Christ was offered once to bear the sins of many. To those who eagerly wait for Him He will appear a second time, apart from sin, for salvation.
Here is a list of those who were baptized by Mormons, feel free to add to my list.
Hitler, and most the jews that died in the camps, Anne Frank, Buddha, all the Catholic popes, Shakespeare, Einstein, Elvis Presley, Marilyn Monroe, Joan of Arc, Josef Stalin, King Herod, Al Capone, Carl Sagen, Charles Schulze, Simon Wiensenthal the nazi hunter, and (yes it showed up on church records briefly) Mickey Mouse