Acts 18v25

This man was instructed in the way of the Lord; and being fervent in the spirit, he spake and taught diligently the things of the Lord, knowing only the baptism of John.

This verse takes place after Paul had spent a good while in Corinth (v18) and then departed with Aquila and Priscilla. When Aquila and Priscilla reached Ephesus (presumably having left Paul at some point along their journey), they met a Jew called Apollos. This verse and the ones immediately before and after it tell us quite a few things about him:

He was an eloquent man, able to speak well (v24)
He was very well learned in the scriptures (v24)
He was fervent in spirit (this verse)
He preached and taught in the synagogue (v26)
He taught ‘the things of the Lord’ – pretty much as Jesus might have taught in similar circumstances (this verse)

Despite all of this, he was still lacking in one area – he only knew of the baptism of John, as this verse tells us. Repentance from sin and John’s baptism are wonderful, but Aquila and Priscilla saw the need to take him aside, where they “expounded unto him the way of God more perfectly.” (v26)

After his meeting, Apollos continued on his journey, going on to help the saints in Achaia. He convinced and converted Jews mightily and publicly, a major addition to his teaching being “shewing by the scriptures that Jesus was Christ.” (v28) Clearly the baptism of John was no longer sufficient, and a belief that Jesus is the Christ and promised Messiah is now also part of the salvation of the believer.