Most Christians will answer this question in one of two ways:
Both of these are correct statements, but neither is the right answer to the question. Baptism is the rite of initiation into the church. In the New Testament, baptism and conversion were so closely associated that one infers the other. When Jesus said to baptize he was not saying that because of his own baptism, which was a special, Messianic baptism that was done "to fulfill all righteousness" (Mt. 3:15). Rather, he was saying that people were to be brought into the church--a process that was synonymous with baptism. It is for this reason that the doctrines of baptism and church are closely related.
Well before the coming of Christ, baptism had been established as the consummating step of the process by which a prosyelite would enter the Jewish faith. The people of the New Testament era were therefore quite familiar with the practice. Note that when the priests and Levites confronted John the Baptist (Jn. 1:19), they did not ask him "What are you doing?", but rather they asked him "Why do you baptize?" When an outsider confessed a faith in Judaism, he would be
Immediately as he came out of the water, he would be given all rights and privileges of Judaism.
Since Christianity began as a Jewish sect, the process by which a Gentile would become a Christian followed this procedure precisely except that circumcision was no longer required (Acts 15:19). Note that this change tended to erase any distinction between men and women, making it clear right at the time of a candidate's initiation that Christianity completely removes the three traditional barriers between people that are enumerated in Gal. 3:28. This verse, believed by many scholars to be a quotation of a baptismal formula in the early church, says "There is neither Jew nor Greek, slave nor free, male nor female, for you are all one in Christ Jesus."
Therefore, functionally, baptism is simply the rite of initiation of in individual into the community of faith. Just as with its Jewish antecedent, it must be voluntary and it must be witnessed. It may be administered only after a confession of faith and instruction (catechism).
In Rom. 6, Paul added a rich theology that explained baptism in terms that were easy enough for first-century Romans to understand, but which are often difficult for modern westerners. In Rom. 6:4, Paul says that "we were therefore buried with him through baptism into his death." This passages suggests that time can be telescoped--that the contemporary believer can actually exist in some sense during Jesus' crucifixion. Furthermore, Paul says that the believer, as he is being baptized, actually participates in the death of Christ; that participation is the basis for Christ's future participation into the believer's death, leading him or her into eternal life.
One should be cautious not to over-extend this point; Paul was not saying that a person converted on his deathbed and who dies before he can be baptized would not achieve salvation in Christ. Such an understanding would give pelagian (magic) overtones to Baptism. But the act of entering the water and then coming out of the water was seen as symbolizing Jesus' death and burial, followed by his resurrection. Because of this symbolism, many evangelicals (especially Baptists) insist that Baptism must be a full immersion. Also, the Greek word transliterated "baptism" in our English Bibles (baptizo) actually means "immerse," but King James insisted on transliterating the word to avoid sending the wrong message to the masses. In a way, however, King James had a point. The ancient church document known as the Didache (the "teaching," c. 70 - 400 AD?) suggests that the early house churches had a great deal of flexibility in how to actually perform the rite.
It is interesting that Joseph Fitzmyer, in is excellent commentary on Romans, affirms the symbolism of Rom. 6 regarding baptism even though his own denomination (Roman Catholic) practices sprinkling rather than full immersion. The baptism of the first Anabaptists in 1525 was done by pouring, although the Anabaptists moved toward full immersion very early. In the end, the "mode" of baptism must remain a matter for each house church to decide.
Infant baptism was a hot button for the historical anabaptists and baptists. It was seen as the very symbol of church-state collaboration, and many of the early confessions anathematize the practice without mincing any words. Modern house church theologians, however, are not nearly as dogmatic on this issue--some are, and some are not. One can take the theology and prophetic symbolism of adult, immersion baptism and yet use "confirmation" as the adult confession of faith. So the material in this section is concerned with the theological basis of baptism--it is not intended to be taken as saying that one must renounce one's infant baptism and be re-baptized as an adult in order to be a Christian. Each must work out his salvation with "fear and trembling" on this point and Christians must learn to respect the decisions of their Christian brothers and sisters when they decide differently in the spirit of Rom. 14:13-23.
That said, let us venture into the biblical case for and against infant baptism. Many Christians, acting on the kerygmatic "the Church is the new Israel," have said that Christian Baptism is equivalent to the circumcision of ancient Israel. As such, it should be performed on infants when they achieve the age of eight days. Further, the water is seen as a symbolic act of "washing" that removes the "original sin" of Adam that was passed on to the infant through inheritance.
Many house church theologians have trouble with this line of reasoning for these reasons:
Those who reject infant baptism often substitute infant "dedication," pledging the church's support of the infant's family during the child's upbringing. They will defer actual baptism until the child reaches the age of moral apprehension. Prior to that age, the child is regarded as "innocent" of sin, as Paul articulates in Romans:
Those who are convinced that infant baptism is a biblical practice are wise to have adult catechism and "confirmation" to signal full participation at the Lord's table.
House church theology attaches no importance to the administrator of baptism. Other than the need for the person administrating the baptism to be a member of the church (that is, someone who is baptized), there is no biblical ordination required. Baptism is simply a voluntary, symbolic act of initiation into the community of faith. Theologically, it is an act that is performed by God, who is always in the role of initiator.
Baptism is an act that need only be performed once. When a believer leaves a church to join another, there is no reason to repeat the baptism. But the Lord's Supper is generally regarded as being reserved for members of the church, which means baptized persons.
Baptism does share one aspect of Jewish circumcision--it is an act of obedience. Just as God commanded the Patriarchs to be circumcised, Jesus commanded the church to "go and make disciples of all nations, baptizing them ..." (Mt. 28:19). This so-called "Great Commission" is not limited to baptizing, however--the church is also to disciple and teach. New Testament baptisms were never performed without first teaching the candidate the biblical essentials (except in the case of Jews, who were regarded as being already trained), and, after baptism had been performed, training continued so that disciples would behave properly when it was their time to take a seat an their place at the table of the family of God.