The unitarians in Ephesus were not regenerated by "John's baptism"
Now when Paul later returned to Ephesus, he found those unitarians there — with their claim to have received "John's baptism." It is obvious that these men — ignorant of the Holy Spirit, and hence also of the Christ alias the One anointed by that Spirit — had been altogether uninfluenced by the Scripture-quoting and Spirit-filled Christian preaching of the instructed Apollos. Indeed, those non-trinitarians only seem to have arrived in Ephesus — after the trinitarian Apollos departed thence, for Corinth.269
It had been some twenty-five years since the death of that great trinitarian, John the Baptizer. The non-trinitarians in Ephesus claimed to Paul that they had been initiated "into John's baptism." Yet they were apparently quite ignorant even about the very existence of the Holy Spirit (and perhaps even of the Lord Jesus Himself)!
That seemed very surprising. For John the Baptizer himself, while baptizing people with water, had always pointed his baptizees (and prospective baptizees) away from himself — and toward the coming Messiah (Jesus Christ). John had always told them how that Spirit-anointed One would soon Himself baptize them — not (once or again) with water, but indeed with His Holy Spirit.270
The unitarians in Ephesus, however, not even alleged they had received their 'baptism' by or from John himself. They only claimed — and that claim itself is suspect! — to have been initiated "into John's baptism."271 Indeed, they frankly admitted to Paul they had 'never even heard whether there is “a holy spirit”' (sic)!272
Clearly, this Spirit-less "John's baptism" these unitarians alleged to Paul they had received — even if it had indeed been administered to them — had not been administered by John himself. For John had been a Spirit-filled person (even from his mother's womb). Indeed, also after growing up, John still testified about the Holy Spirit during his Spirit-filled preaching — and also while baptizing! Hence, this 'Spirit-less' rite referred to in Acts 19:3b, was not Johannine! Indeed, it had started to be administered probably only after John's own death.
For it seems that certain unitarians had then started initiating people "into John's baptism." By this, they probably meant they were initiating 'in the name of John' or perhaps even 'into the name of John' — neither of which John himself would ever have done! Very clearly, this 'Spirit-less' rite was certainly not the Christian baptism John himself had administered — to those who thereafter soon became the disciples of Jesus Himself (without then ever being 'rebaptized' by Jesus or by anyone else).
Yet this 'Spirit-less' water-rite which the unitarians in Ephesus claimed to have received before they met Paul — the rite they called "John's baptism" — had clearly not regenerated them! For, even long thereafter, they had 'never even heard whether there is “a holy spirit”' (sic)!
However, John himself had spoken quite clearly about the Holy Spirit — both before and while baptizing. Indeed, before administering that water-rite, he had urged his candidates to repent (or to turn to God) and to believe in the coming Christ. Consequently, the baptisms administered by John did not themselves regenerate.
Even more interesting. Although the unitarians at Ephesus indeed claimed to have been baptized "into John's baptism," they never claimed that the Holy Spirit had regenerated them through that water-rite. To the contrary. They readily admitted they had never even heard 'whether there is a holy spirit.' So they were then admitting that, in spite of their allegedly having received "John's baptism," they had still not yet been regenerated.