The ritual of baptism is prefigured in the purification rites of Jewish law and tradition. In the Tanakh and tradition of the teachers of the Torah, a ritual bath for purification from uncleanness used to be required under specified circumstances, in order to be restored to a condition of ritual purity. For example, women after menses, and after a number of blood-free days following child-birth, were washed in a ritual bath, called a mikvah. Those who became ritually defiled by contact with something infectious, would also use the mikveh as part of their healing. Washing was also required for converts. Through practices such as these, immersion in the mikveh came to represent purification and restoration, and qualification for full religious participation in the life of the community.
He went into all the country around the Jordan, preaching a baptism of repentance for the forgiveness of sins. As is written in the book of the words of Isaiah the prophet: "A voice of one calling in the desert, 'Prepare the way for the Lord, make straight paths for him. Every valley shall be filled in, every mountain and hill made low. The crooked roads shall become straight, the rough ways smooth. And all mankind will see God's salvation.'"
Produce fruit in keeping with repentance.
John declared that repentance was necessary, prior to forgiveness. There must be a return to God. This implies that the stain of sin is not ineradicable, but can be removed by putting off polluting acts and returning to the way of the Lord, all of which was symbolized in his baptism.
The Christians believe that John also taught that his baptism was not finally sufficient, and that repentance would not attain to its goal of separation from sin, apart from a greater baptism which it was not in his power to give. According to the Gospel of Luke, John taught, "I baptize you with water; but one comes who is stronger than I, of whom I am not worthy to untie the strap of his sandals; he will baptize you with the Holy Spirit and fire; his winnowing fork is in his hand to clean out his threshing floor and to gather the wheat into his storehouse, but the chaff he will burn with inextinguishable fire." (Luke 3,16-17)Christians believe that John's baptism shows that the effort to make oneself acceptable to God by repentance would be superseded, made complete by the coming of the Lamb of God that takes away sins.
According to the Gospel of John, after John baptized Jesus, he testified concerning him, "I have seen the Spirit coming down as a dove from heaven, and it remained upon him. And I had not known him, but the one who sent me to baptize with water, that one said to me, On whomever you see the Spirit coming down and remaining upon him,this is the one baptizing with the Holy Spirit. And I have seen, and I have testified that this is the son of God." ( John 1,32-24)"Behold the Lamb of God, that takes away the sins of the world." From this point on, water baptism became identified with the followers of Jesus, who preached "Repent, for the kingdom of God is near."
One ecumenical statement prepared by representatives across a spectrum of Orthodox, Roman Catholic and Protestant traditions of Christianity, attempts to express a common understanding of baptism, as it is derived from the New Testament.
" ... according to Acts 2, baptisms follow from Peter's preaching and lead those baptized to life in the community: "They devoted themselves to the apostles' teaching and fellowship, to the breaking of bread and the prayers" (2:42) as well as to the distribution of goods to those in need (2:45). Those who heard, who were baptized and entered the community's life, were already made witnesses of and partakers in the promises of God for the last days: the forgiveness of sins and the outpouring of the Holy Spirit on all flesh (2:38). Similarly, in what may well be a baptismal pattern, 1 Peter testifies that proclamation of the resurrection of Jesus Christ and teaching about new life (1:3-21) lead to purification and new birth (1:22-23). This, in turn, is followed by eating and drinking God's food (2:2-3), by participation in the life of the community - the royal priesthood, the new temple, the people of God (2:4-10) - and by further moral formation (2:11 ff.). At the beginning of 1 Peter the writer sets this baptism in the context of obedience to Christ and sanctification by the Spirit (1:2). So baptism into Christ is seen as baptism into the Spirit (cf. 1 Cor. 12:13). In the fourth gospel Jesus' discourse with Nicodemus indicates that birth by water and Spirit becomes the gracious means of entry into the place where God rules (John 3:5)." 
The most commonly cited reference for the command justifying the continuing practice of baptism by Christians, is the "Great Commission," found in the Gospel of Matthew, chapter 28, verses 18-20. It is typically viewed as a means by which a person is joined to Christ and his body, the Church, after which the newly baptized person is considered to be a Christian.
The liturgy of baptism in the Catholic and Eastern Orthodox tradition makes clear reference to baptism as not only a symbolic burial and resurrection, but an actual supernatural transformation, one that draws parallels to the experience of Noah and the passage of the Israelites through the Red Sea divided by Moses. Thus baptism is literally and symbolically not only cleansing, but also dying and rising again with Christ.
The Catholic Church prescribes that in case of emergency any person, even someone not baptized, can baptize, if he has the required intention. The words "N., I baptize you in the name of the Father, and of the Son, and of the Holy Spirit," are said while pouring water three times on the head. The sign of the cross is then made over the recipient. The omission of the name or the sign of the cross and the addition of 'Amen' at the end have no effect on the validity of the sacrament. The validity of Baptism is doubtful if impure water is used. In such a case, the sacrament should be repeated conditionally with certainly valid matter as soon as possible if the emergency persists. (See Moral Theology by Fr. Heribert Jone)
Baptist groups derive their name from the restrictions that they traditionally place on the mode and subjects of the ordinance of baptism. Immersion is regarded as the only legitimate, biblical baptism; and baptism is not administered to children. Those who hold views influenced by the Baptists, may perform the ceremony indoors in a baptismal, a swimming pool, or bathtub, or outdoors in a creek or river: as long as there is water, nothing prevents the performance of Baptism. Protestant groups influenced by these convictions usually emphasize that it memorializes the death and resurrection of Jesus, which by a gift of God has become the principle of repentance and new life for those who have professed belief in Him, symbolizing spiritual death with regard to sin and a new life of faith in God. They typically teach that baptism does not accomplish anything, but is an outward sign or testimony, a personal act, indicating the invisible reality that the person's sins have already been washed away as a result of their profession of faith. Rather than by what they say baptism is, Baptist views are distinguished more by what they say it is not (not for children, not by sprinkling, not a sacrament, not a means of grace).
Priesthood authorities in the Church of Jesus Christ of Latter-day Saints (LDS Church; see also Mormon) baptize only by immersion. The earliest age at which a person may be baptized is eight years, which is considered the age at which children know good from evil and become accountable for their actions. Latter-day Saints (LDS) believe that baptism is only the first of several ordinances required for exaltation and that faith and repentance precede baptism. Typically soon after a person is baptized into the LDS Church a Priesthood authority lays their hands on the head of the newly baptized person during Sunday Sacrament meeting and confirms the person a member of the Church and confers upon the person the "gift of the Holy Ghost". Furthermore, LDS believe that: legitimate baptism can only be given by some one with legitimate Priesthood authority (that is, it must be given by an LDS Priesthood authority); only baptism by immersion is legitimate; and infant baptism is a perversion of Christianity. See Latter-day Saint condemnation of infant baptism.
Membership into the LDS Church is granted only by baptism whether a person has been raised in the Church or not. The LDS Church also practices baptism for the dead along with all other Church ordinances LDS perform vicariously or by proxy in their temples for everyone who has not received these ordinances while living.
Baptisms inside and outside the temples is usually done in a font although they can be perfomed in any large body of water. In the temples the fonts are usually laid out on the sculptures of twelve oxen representing the twelve tribes of Israel. Great care is taken in the execution of the baptism; if the baptism is not executed exactly it must be redone. The person administering the baptism calling the baptisee by name and must state these words exactly: "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." Every part, limb, hair and clothing of the baptisee must then be fully submersed into the water or the baptism must be redone. Two Priesthood authorities stand by as witnesses in part to make sure that the baptism is executed properly.
LDS believe that through repentance and baptism a baptisee is cleansed of all previous sin. The process of repentance and sanctification continues by partaking of the Sacrament every Sunday which LDS consider to be a renewing of the baptismal covenant. They also believe that baptism is symbolic both of Jesus's death, burial and resurrection and of the baptisee's death and burial of the natural or sinful man and rebirth as a disciple of Jesus.
The Sikh baptism ceremony, dating to 1699, was established when the tenth religion's leader (Guru Gobind Singh) baptised 5 followers of his faith and then was baptised himself by his followers, similar to Jesus' baptism by John the Baptist. The Sikh baptism ceremony is called Amrit Sanskar or Amrit Sanchar. The Sikh is said to have taken Amrit once they have been baptised. In Sikhism, the baptised Sikh is also called an Amritdhari literally meaning Amrit Taker or one who has Taken on Amrit.