...and I believe in one baptism for the forgiveness of sins. So says our Nicene Creed, and so says our Bible. "There is one Lord, one faith, one baptism." (Eph. 4:5) "Whoever believes and is baptized will be saved." (Mark 16:16) "Be baptized every one of you in the name of Jesus Christ for the forgiveness of sins." (Acts 2:38) "Rise and be baptized, and wash away your sins, calling on his name." (Acts 22:16)
It may seem strange that Jesus, who himself forgave sins without any form of ceremony, should have sent his disciples out to ritually baptize people "in the name of the Father, and of the Son, and of the Holy Spirit" (Matt. 28:19). But it is very clear that baptism was for those who could not meet Jesus personally - Jesus himself, John tells us, "did not baptize" (John 4:2) (or if he did, then it was "with the Spirit", as John the Baptist had predicted).
We have a word for this kind of ritual: sacrament. A sacrament is not just a sign of God's redeeming grace - it is a pledge for it, a guarantee, a way for us to identify ourselves with Jesus. "We were buried with him by baptism into death, so that as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life. For if we have been united with him in a death like his, we shall certainly be united with him in a resurrection like his." (Rom. 6:4-5)
So in baptism we share in Jesus' death and resurrection, and in the early church, new Christians were baptized at Easter, to share fully in the joy of new life in Christ. The water of baptism symbolized two things. On the one hand, it is the water of death (as in Ps. 69, for instance), the chaotic water which covered the face of the earth at creation, the water which destroyed humanity in the days of Noah - see 1 Pet. 3:20 (there are good grounds for thinking the whole of this letter was written as a sermon for a service of baptism!). But on the other, it washes and cleanses us, and takes away the dirt. Both of these aspects are part of our own baptism.
Believing in "one baptism" raises some extra questions. If there is only "one baptism", it does not matter who administers it. The fellowship of baptized Christians transcends the barriers of our divisions and denominations. In the last analysis, we are all apostles, sent to bring Christ to the world outside, though because there is also "one body", baptism normally takes place within that body, part of a journey shared with the whole family of the church.
If there is only "one baptism", we cannot be baptized twice. There was a time when people delayed their baptism as long as possible, heedful of the awful warning in Hebrews 6:4, "it is impossible to restore again to repentance those who have ... shared in the Holy Spirit". (Taking single verses out of context is never a good idea: six verses later, the writer assures readers that God will not allow this unreasonable punishment to happen!) From the earliest times, whole families were baptized, however, including, we must assume, slaves and children. Little children certainly have scope for growing in faith, but then so do we all, and there are no stages in baptism - no primary certificate, secondary certificate, Maturität or doctorate!
The two aspects of the water mean there are two possible ways to baptize - by submersion (the baptistery beside the Rhine at Kaiseraugst will give you an idea of how this was done, in a deep pool) to symbolize a rescue from drowning, or by immersion - a simple washing to symbolize our cleansing from our sins. We must assume (from Acts 2:41, for example) that the first was an ideal, the second a practical substitute!
What happens to the unbaptized? The western church devised the concept of limbo, a godless void, to which those who had never known Jesus were consigned. Yet God saves whoever God wishes to save - it is not for us to judge or to sort people into categories. What remains certain is that in baptism, we share in a new life, and take a step forward on our Christian pilgrimage - one step nearer to heaven and to God's kingdom.
HD