{With the baptism of repentance} (aptisma metanoias).
Cognate accusative with ebaptisen and the genitive metanoias
describing the baptism as marked by (case of species or genus),
not as conveying, repentance just as in  Mr 1:4  and that was the
work of the Holy Spirit. But John preached also the baptism of
the Holy Spirit which the Messiah was to bring ( Mr 1:7f.  Mt
3:11f.  Lu 3:16 ). If they did not know of the Holy Spirit, they
had missed the point of John's baptism. {That they should believe
on him that should come after him, that is on Jesus} (eis ton
erchomenon met' auton hina pisteus(9373)in, tout' estin eis ton
I(8873)oun). Note the emphatic prolepsis of eis ton erchomenon met'
auton before hina pisteus(9373)in with which it is construed. This
is John's identical phrase, "the one coming after me" as seen in
 Mr 1:7  Mt 3:11  Lu 3:16  Joh 1:15 . It is not clear that these
"disciples" believed in a Messiah, least of all in Jesus. They
were wholly unprepared for the baptism of John. Paul does not
mean to say that John's baptism was inadequate, but he simply
explains what John really taught and so what his baptism
signified.