Water is so universally recognized both as an essential sustainer of Life and as an agent of cleansing that it is hardly surprising that from earliest times it has been used as a religious symbol. It is commanded in the OT (e.g. Lev.8:6). By the time of Jesus it had become customary for non-Jews wishing to become adherents of Judaism to be baptized as well as circumcised. These proselytes were then said to be 'born again'.
As the convert plunged under the water he died to his old Life and on rising began a new life under God's law. John's baptism can be understood thus. It was a preparation for entering the Messianic kingdom. It was a one-off penitential rite which placed the recipient in as close a relationship with God as was possible under the old covenant. John looked forward to the coming of him who would 'baptize with Holy Spirit and fire.' Those who received this would be forgiven and have power for the future new life as a person living fully in God's kingdom.
Jesus himself underwent John's baptism. Although this may have been initially understood as his incorporating himself into this new life community, it was also seen as marking the commencement of his earthly ministry. Jesus then in his turn is seen to command his disciples to baptize those who accept his message, thus becoming also members of the community of faith. 'Go therefore and make disciples of all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost.'(Matt.28:19). There are doubts as to whether this verse was part of the original text, but the belief that Christian baptism originated from the Lord himself does not depend on this alone. The whole NT record shows baptism as the sale means of entry into the Body of Christ. Baptism in the name of the Trinity is the universal form, although there are suggestions in the NT that occasionally it was administered in Jesus' name alone. This practice may have lasted up to the 4CAD on the basis that the name of one Person of the Trinity could imply God as a whole.
Baptism, as the sacrament of initiation into the Church, can be seen as the outward form and vehicle for entry into Christian life. It is a work of the Holy Spirit to which the individual responds by faith. We receive the Spirit in baptism which incorporates us into the Church which is the community of the Spirit. We are re-born into a new family, a family in which we can know God as Father. There is a sense in which we can call the Lord our brother, so it is appropriate that in this service we should receive our Christian name. Baptism is the beginning of a lifelong process which here commences with a Symbolic turning away from evil and a turning towards the life of grace and reconci.1iation which we are offered in and through Jesus Christ. The water of baptism is significant at this point as an agent of purification. The process of sanctification has begun. We have been given the spiritual tools for the job.
The most dramatic description of all this comes from S. Paul when he speaks of himself being buried and rising again with Christ, that is, participating in his death and resurrection. We are 'baptized into his death' and raised to 'newness of life', as was Christ. (Rom.6:4).
With reference to my earlier comment that the response of faith is a necessary part of baptism; this has led to questions as to the reality of infant baptism. In this situation of course the necessary response of faith is initially that of the parents and godparents. There seems no doubt that the practice was normal in apostolic times. We have the record of the baptism of whole households (Acts 16:15; 1 Cor. 1:16). S. Peter can bid people repent and be baptized, 'for to you is the promise and to your children' (Acts 2:39). It is in full accord with Jewish contemporary ideas of religion. Jesus himself was brought into the community of faith on the eighth day of his life. Further, infant baptism embodies a profound spiritual principle. Christianity starts not with what we do for God but with what God does for us.
We do not have to climb up to God first, to earn his goodwill by so much repentance, or faith or so many good works. God's love and his free gifts to us come first. Privilege comes before responsibility. The reverse view is the human view, and is a constant danger in religion - but in practice it always leads to a hard and gloomy view of God. The Prodigal Son had not earned his position as son. He had received it, and used it unworthily. But in the hour of distress and realization he could fall back on it, and be forgiven and reconciled with his father. Even though much of the consequence of his wrongdoing would remain with him always, he was restored to fullness of relationship.