...had exhorted the Jews to lead righteous lives, to practise justice towards their fellows and piety towards God, and so doing to join in baptism. In his view this was a necessary preliminary if baptism was to be acceptable to God. They must not employ it to gain pardon for whatever sins they committed, but as a consecration of the body implying that the soul was already thoroughly cleansed by right behaviour.
(Antiquities 18.116-119)
Precisely the reason the consensus considers genuine the Baptist passage:
One of the points on which there is disagreement between this passage and the Gospels is the nature of John’s baptism. In Mark and Luke, at least, this is a “baptism of repentance for the forgiveness of sins.”
(Mark 1:4)
(Luke 3:3)
John the Baptist appeared in the wilderness preaching a baptism of repentance for the forgiveness of sins.
(Mark 1:4)
And he came into all the district around the Jordan, preaching a baptism of repentance for the forgiveness of sins;
(Luke 3:3)
http://peterkirby.com/john-the-baptist-authentic.html
...well, precisely just that reason (!), was the real goal of the Judaizing interpolator of the Baptist passage:
1) The Gnostics (=haters of YHWH) identified the baptism of water with the baptism of nous (spirit), basing on the literal interpretation of the Dialogue on the krater in Corpus Hermeticum 4:4-6. Water coincides stricto sensu with Spirit.
2) against the Gnostics, the baptism of water has to be disconnected from the baptism of Spirit:
- John will baptize only by water
- Jesus will baptize with water and Spirit.
It was emptied of any Spirit's effect, because it had - in the mind of the interpolator - be emptied of any Spirit's effect.