John Gill 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)

Then said Paul In reply to their answer, understanding them that they were baptized by John, he takes it up, and gives an account of John's baptism: showing how agreeable it was, and that it was the same baptism with the baptism of Christ, being administered in his name:

John verily baptized with the baptism of repentance ; which required repentance antecedent to it, and was a fruit and effect, and so an evidence of it:

saying to the people ; the people of the Jews, the common people, the multitude that attended on his ministry:

that they should believe on him, which should come after him , that is, on Jesus Christ; so that he preached faith in Christ, as well as repentance towards God; and made the one as well as the other a necessary prerequisite to baptism; which shows, that his baptism and Christian baptism are the same.