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Much of the theology of Mormon baptism was established during the early Latter Day Saint movement founded by Joseph Smith, Jr. According to this theology, baptism must be by submersion for the remission of sins (meaning that through baptism, past sins are forgiven), and occurs after one has shown faith and repentance. Mormon baptism does not purport to remit any sins other than personal ones, as adherents do not believe in original sin. Mormon baptisms also occur only after an "age of accountability" which is defined as the age of eight years. The theology thus rejects infant baptism. According to the account in Joseph Smith—History 1:68, the first Mormon baptisms occurred on May 15, 1829, when Joseph Smith and Oliver Cowdery baptized each other in the Susquehanna River near Harmony, Pennsylvania shortly after receiving the Aaronic priesthood from John the Baptist.
In addition, Mormon theology requires that baptism may only be performed with one who has been called and ordained by God with priesthood authority. Because the churches of the Latter Day Saint movement operate under a lay priesthood, children raised in a Mormon family are usually baptized by a father or close male friend or family member who has achieved the office of priest, which in Mormonism is conferred upon worthy male members at least 16 years old.
Baptism is seen as symbolic both of Jesus' death, burial and resurrection and is also symbolic of the baptized individual putting off of the natural or sinful man and becoming spiritually reborn as a disciple of Jesus.
Membership into a Latter Day Saint church is granted only by baptism whether or not a person has been raised in the Church. Most Latter Day Saint churches do not recognize baptisms of other faiths as valid because they believe baptisms must be performed under the church's unique authority. Thus, all who come into one of the Latter Day Saint faiths as converts are baptized, even if they have previously received baptism in another faith.
When performing a baptism, the following instructions are followed:
"The person who is called of God and has authority from Jesus Christ to baptize, shall go down into the water with the person who has presented himself or herself for baptism, and shall say, calling him or her by name: Having been commissioned of Jesus Christ, I baptize you in the name of the Father, and of the Son, and of the Holy Ghost. Amen.
"Then shall he immerse him or her in the water, and come forth again out of the water."
Baptisms inside and outside the temples are usually done in a baptismal font, although, baptisms done outside the temples can be performed in any body of water in which the person may be completely immersed. The person administering the baptism must recite the prayer exactly, and immerse every part, limb, hair and clothing of the person being baptized. If there are any mistakes, or if any part of the person being baptized is not fully immersed, the baptism must be redone. In addition to the baptizer, two priesthood holders witness the baptism to ensure that it is performed properly.
Baptism for the dead 
After the death of the movement's founder, Joseph Smith, Jr. in 1844, rebaptism became an important ordinance in a faction of the church led by Brigham Young. Young led his group to the Great Basin in what is now Utah, and most of his followers were rebaptised not long after entering the basin as a sign that they would rededicate their lives to Christ. During the "Mormon Reformation" of 1856–1857, rebaptism became an extremely important ordinance, signifying that the church member confessed their sins and would live a life of a Latter-day Saint. Church members were rebaptized prior to new covenants and ordinances, such as ordination to a new office of the priesthood, receiving temple ordinances, getting married, or entering plural marriage.
Rebaptism remains a practice in The Church of Jesus Christ of Latter-day Saints, but today, it is practiced only when a previously excommunicated member rejoins the church. In such cases, the wording of the ordinance is identical to that of the first baptismal ordinance.
See also 
- Porter, Bruce D. (October 2000). "The First Principles and Ordinances of the Gospel". Ensign. Retrieved March 24, 2009.
- See Doctrine and Covenants 68:25, 27.
- See Book of Mormon, Moroni 8:4-23.
- Joseph Smith—History 1:68
- See, e.g., Guide to the Scriptures: Baptism, Baptize, §Proper authority.
- See, e.g., Gospel Topics: Priest.
- See, e.g., Bible Dictionary: Baptism, ¶2.
- Doctrine and Covenants 20:73–74.
- Duties and Blessings of the Priesthood: Basic Manual for Priesthood Holders, Part B: Performing Priesthood Ordinances, §Baptism.
- Duties and Blessings of the Priesthood: Basic Manual for Priesthood Holders, Part B: Performing Priesthood Ordinances, §Confirmation.
- Baptisms for the Dead Baptisms for the Dead from official LDS website