Fringe fanatics after the death of John the Baptizer
Also to us ourselves — as previously to the great Rev. Prof. Dr. Abraham Kuyper Sr.120 — it indeed seems that, at an earlier time than Paul's Acts 19 return to Ephesus, "some" of the followers of John had become confused. This probably occurred only after John himself had temporarily questioned Jesus' Messiahship, and especially after John's own violent death. Matthew 11:2f,11f & 14:10-12.
Instead of then following Jesus, those confused followers of John had then started trusting in falsehoods. Thereafter, it seems even they themselves had then started teaching erroneous doctrines. Unlike John himslf (Mark 1:4-11), they were certainly not trinitarians. Like the Judaists who had apostasized from Old Testament Trinitarianism, they too were doubtless unitarian. Yet unlike those Judaists they also seem to have begun to deify even John himself, at least incipiently.