A.T. Robertson Commentary Acts 19:4

"And Paul said, John baptized with the baptism of repentance, saying unto the people that they should believe on him that should come after him, that is, on Jesus." — Acts 19:4 (ASV)

With the baptism of repentance (βαπτισμα μετανοιας). Cognate accusative with εβαπτισεν and the genitive μετανοιας 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.; Matthew 3:11f.; Luke 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 (εις τον ερχομενον μετ' αυτον ινα πιστευσωσιν, τουτ' εστιν εις τον Ιησουν). Note the emphatic prolepsis of εις τον ερχομενον μετ' αυτον before ινα πιστευσωσιν with which it is construed. This is John's identical phrase, "the one coming after me" as seen in Mr 1:7; Matthew 3:11; Luke 3:16; John 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.