Mormons are likely the only religion that still practices the Biblical ordinance of baptism for the dead.
From Peter, in the New Testament, we learn that the gospel is preached in Heaven to those who did not receive it on earth, so they can be fairly judged:
For for this cause was the gospel preached also to them that are dead, that they might be judged according to men in the flesh, but live according to God in the spirit (1 Peter 4:6).
This is necessary because many never have the opportunity to hear the gospel at all in their lifetimes and others heard, but did not receive a witness of it in their lifetime. It is not a second chance, but a first chance. God, who is loving and fair, does not punish people for things beyond their control.
The Bible teaches us that baptism is necessary for salvation.
Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God (John 3:5).
Unfortunately, those same people who never heard the gospel of Jesus Christ also had no opportunity to be baptized. Some who were baptized were baptized by people without the proper priesthood authority. Because God is fair—and He chose the time and circumstances of each birth—he would never condemn one of His own beloved children without a reasonable opportunity to hear and decide about the gospel. He would not deny them baptism simply because they’d never had an opportunity to receive it.
Of course, baptism, is an earthly ordinance and requires a body. It is impossible for the newly dead to receive baptism and so it must be done vicariously. Mormons perform this ordinance in the Mormon temples, and unlike most temple ordinances, it can be done by older teenagers.
Baptism for the dead is mentioned in the Bible:
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? (1 Corinthians 15:29).
Paul was helping the Corinthians to understand the resurrection of Jesus Christ. Clearly he would not use an apostate ordinance to prove a great truth, so we can see there were people doing baptisms for the dead appropriately and that doing so demonstrated the dead were indeed resurrected.
Receiving this vicarious ordinance does not force people to become Mormons. A person who meets with the Mormon missionaries on earth is invited to be baptized. He is free to accept or to decline. Agency—the right to choose—is an essential part of Mormon belief. A forced conversion is not a conversion at all.
When a person dies, he or she is taught the gospel, as we saw in 1 Peter. One year after the death, the person’s family may choose to perform a baptism vicariously in that person’s name. We have no way of knowing their reaction to being taught the gospel, and therefore Mormons simply perform the baptism and the person is free to accept or reject the ordinance. If the person rejects it—because even those receiving it in Heaven and knowing it is true may decide they’d rather not be bound by the gospel—it is as if it never happened at all. Mormons record only that the opportunity was given. The person is never listed as a member of the Church—that is for God to handle Himself.
Baptisms for the dead are evidence of God’s great love for us. Mormons believe that everyone who ever lived is God’s child. Although many churches teach that we are God’s children only by adoption when we become Christians, Mormons know the scriptures use the term in two ways. We are all God’s children by virtue of having been created by Him:
We are the offspring of God: Acts 17:29;
Ye are gods, children of the most High: Ps. 82:6;
In the other sense, we are God’s children by virtue of the accepting the atonement of Jesus Christ. However, we are all His and He loves each and every one of His. He wants us to return home to us and He is infinitely fair. For this reason, He will never arbitrarily punish us for things that were beyond our control. Baptism for the dead is one of the ways the atonement makes the world fair—while mortality may not give us all the same opportunities, God’s plan gives us all an equal opportunity to be saved.