The antecedents to Christian baptism appear to be developments of baptistic practices among some Jewish groups, themselves an development of some of the 'washing for purity' rites in the OT. Thomas, in Le Mouvement Baptiste en Palestine et Syrie says he discovered no evidence of 'pagan baptists', that the phenomenon was purely Jewish. We also know that the Qumram community, among others, practised ritual washings, though it is not clear if they can be seen as truly parallel. Baptism's distinct feature is that it is an initiatory rite undertaken once for all.

There has also been the suggestion, and argument, that in the 1st century Jews practiced proselyte baptism, that is they baptised outsiders who were becoming Jews. Problematically, neither Josephus, Philo, nor the NT seems to mention such a practice. The first clear references to proselyte baptism that I know of are in Epictetus (Dissert. II.9.9-21), also a possible reference in the 4th Sibylline Oracle, and then in the Mishna (b.Yebamot 46A).

If proselyte baptism were known earlier than our records indicate, then John the Baptist's novelty is in asking Jews, who were not proselytes, to undergo baptism as if they were. Essentially it is a statement of the need to become an insider as if one was an outsider, and so an act of repentance/conversion/cleansing.

The early church takes up that practice and moves it in a slightly different direction, but one clearly built on and from John the Baptist's practices. Jesus, through his disciples, seems to baptise disciples of himself, and teaches the practice as an initiatory rite with the same kind of meaning. The practice of early christian baptism, as seen in the Didache for example, seems patterend on Jewish practice (cf. the tractate Mikwaoth), but the baptismal theology begins to shift, though with parallels. For example, the NT links baptism with a dying and rising experience that symbolises new birth (Romans 6:1-6), which may fine antecedents in (b.Pesahim 91B, b.Yebamot 22A), however baptism entirely dislocates the place of circumcision in the rite of people becoming Christians, rather than Jews, and the symbolism of death and rebirth is not abstract, but tied to participation in the historical death and resurrection event that Christians believed.

What is it's role? It was the definitive rite which marked the entry of a believer into the Christian community, integrating meanings of participation in Jesus, repentance from sin, and cleansing from unholiness. It was probably built on Jewish practices, but especially the innovations of John the Baptist, and developed by Jesus and his early followers in a new direction.