What really separated Christianity and Judaism in the beginning was not any positive theological doctrines. I should like to propose that a man named Jesus coming back from the dead, being a son of God, or a prophet, or a magician with powers from God were all insignificant later issues. It seems to me that the original issue was circumcision.
The Baptism ritual may be seen originally as a gimmick to get gentiles interested in Judaism. It is a nice cleansing ritual developing out of utensil cleansing rituals. If the body is a utensil that you use like a knife or spoon, why not wash it too and bless it with a prayer?
This is a very nice gimmick for getting the others interested in Judaism and presenting it as a healthy practice. However, the real mark of Judaism is the cutting of the foreskin. In order to become Jewish you have to be circumsized. That’s the mark or the sign that you are a Jew. It is the physical proof of your agreement to the Hebrew covenant with God and all other Jews.
The Jewish revolt against the Romans in the First century needed help. Romans probably outnumbered Jews at least 10 to 1. Therefore the Jews needed help to free themselves from the heavy taxation and political control of the Romans. They needed to recruit Jews from outside the ranks. Increasing the number of Jews was a strategy for liberation. It was purely a strategic move to serve a political goal.
The problem was that it was easy to attract people’s attention. Judaism had a beautiful temple, the idea of an invisible or unknown God who would protect you, and it had the pleasurable watery baptism ritual. The problem was that few gentiles would want to take the next necessary step after baptism of cutting off the foreskin of their penis. (Note: women did not count for much in ruling class eyes at this time in Judea, so recruitment policy would only aim at men)
Christianity begins with the more practical militant Jews who don’t care about the foreskins and Jewish tradition. For them the baptism ritual is enough. Their attitude is “Dunk them under the water and they’re in, they’re one of us. They can fight on our side.” They can argue that if the Jews make someone a king by anointing them with a little oil on the head, can’t just dipping someone under the water make someone a Jew? Is not all things possible with God?
This is the founding idea of Christianity. the idea that baptism can be a substitute for circumcision. It represents the most important and earliest argument in the writings of Paul and it is symbolically represented in the Baptism of Jesus, where a Galilean gentile suddenly gets magical powers and becomes a Jew (a son of God) simply by undergoing the Baptism ritual. (Making Jesus a Jew to begin with is a Second century afterthought
Christianity begins with Jews who want to devalue the circumcision ritual and add value to the baptism ritual.
In a certain sense, the transformation of the baptism ceremony into a full initiation ceremony is all that Christianity was in the beginning and the rest of Christian history is footnotes.