They said to him, “No, we haven’t even heard that there is a Holy Spirit.”
3 He said, “Into what then were you baptized?”
They said, “Into John’s baptism.”
4 Paul said, “John indeed baptized with the baptism of repentance, saying to the people that they should believe in the one who would come after him, that is, in Jesus.”
5 When they heard this, they were baptized in the name of the Lord Jesus.
(Acts 19:1-5)
Really, I am beginning to suspect that the great "secular" assumption about John the Baptist being "not christian but christianized" is part and parcel of the diffuse judaization old and new in matter of Christian origins.
It is evident that the author of Acts, a mere propagandist, wants to make us believe that the "Baptism of John" was disconnected from Jesus. The only link conceded by him is that John predicted Jesus. John predicted Jesus, "therefore", as the logic/propaganda goes, John didn't know Jesus.
The hermeneutic of suspicion says me that the contrary was probably true. The "Baptism of John" was connected someway with Jesus.
Only that embarrassing connection can explain Acts 19:1-5.
Only that embarrassing connection can explain Christian interpolations of John the Baptist in Josephus where John is a "Gospel" John without apparently no connection with Jesus at all.