Baptism serves as an initiation of purification for most Christian churches. In earlier times, it was considered an essential act in order to receive salvation, and it is a sacrament which
conveys divine grace, blessing, or sanctity upon the believer who participates in it, or a tangible symbol which represents an intangible reality.
It is often symbolic of cleansing of sins, or even of original sin, a reason that infant baptism, or christening, is so popular. It is also symbolic of dying and a rebirth with Christ, and baptism is also tied to the acts of both burial and resurrection. Some churches acknowledge that baptism represents a supernatural transformation symbolic of Biblical events such as that of Noah and the great flood, and Moses' parting of the Red Sea that allowed the passage of the Israelites.
According to Martin Luther,
To put it most simply, the power, effect, benefit, fruit, and purpose of Baptism is to save. No one is baptized in order to become a prince, but as the words say, to "be saved". To be saved, we know, is nothing else than to be delivered from sin, death, and the devil and to enter into the kingdom of Christ and live with him forever.
Christian baptism is a religious rite of washing with water as a sign of purification and consecration and sometimes initiation. In the Old Testament, baptism signifies purification from sin and devotion to God. For the Roman Catholics, babies receive this first of seven sacraments, the Sacrament of Baptism, which signifies the giving of grace to remove original sin. The baby's head is anointed with oil and holy water is poured upon the forehead. This rite indicates that the baby is brought into the Catholic faith and, as a result, can go on to receive the other sacraments and participate in all that the faith includes. In Protestant religions, babies are not usually baptized; rather the members of the church are baptized when they are saved and then immersed in water, or when they recognize Jesus Christ as their Savior. Again, baptism is an initiation ceremony and a public testimony of the Christian's acceptance and belief in the precepts of his/her church.
The answer depends upon which denomination someone practices. If someone is a Baptist, then baptism is an act that someone chooses to commit--it is not done during infancy. The purpose of Baptism to a Baptist is that it serves as an identification of that person with Jesus Christ and His Church. It demonstrates that the person being baptized has made a profession of faith in the teachings of Christ and the Bible (especially the New Testament).
In contrast, if someone practices Catholicism, baptism is generally performed upon infants or upon those just entering the faith because Catholics believe that humans are spiritually dead (from birth) and that the baptism allows God's spirit to enter the soul and atone it (this is called "sanctifying grace").
In general, baptism is related to practioners believing that they need Christ to atone for their sinful nature, and baptism allows them to either acknowledge that need or fill that need.
Baptism signifies the first step in joining the christian religion
It means that you are showing to everyone: "I am a child of God and am willing to follow him for the rest of my life." It "cleans" you.