Note 2 at Ac 19:3: John's baptism was a baptism with water unto repentance for the remission of sins (see note 4 at Mt 3:11). John preached that the Lamb of God had come to take away the sin of the world (Joh 1:29), but he didn't administer the baptism of the Holy Spirit. However, both John the Baptist (Mt 3:11, Mr 1:8, and Lu 3:16) and Jesus (Joh 7:38-39) prophesied the church age when believers would be baptized with the Holy Ghost.

Therefore, these disciples were men who had been born again through hearing the good news about the Messiah having come to bear mankind's sins, but they had not been baptized with the Holy Spirit. It amazed Paul that they had become disciples without hearing about the baptism of the Holy Spirit. This shows that Paul included teaching on receiving the baptism of the Holy Spirit as a part of the Gospel that he shared.

These disciples were most likely converts of Apollos (see note 1 at Ac 18:24). Apollos did not have the revelation of the baptism of the Holy Spirit (see note 5 at Ac 2:4) until Aquila and Priscilla (see note 2 at Ac 18:2) explained it to him in Ephesus.