Jamieson, Fausset and Brown on Acts 19:4

4. Then said Paul, John ... baptized with the baptism of repentance--water unto repentance.

saying unto the people, that they should believe on him which should come after him--that is, who should baptize with the Holy Ghost. The point of contrast is not between John and Christ personally, but between the water baptism of John unto repentance, and the promised baptism of the Spirit from the hands of his coming Master unto new life. As to all the facts, or at least the significancy, of this baptism, which made the whole life and work of Christ another thing from what it was conceived to be before it was vouchsafed, these simple disciples were unenlightened.