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.
Paul explained baptism to the unregenerate unitarians
Paul now explained,273 to those ignorant unitarians at Ephesus, the nature of the true Christian baptism which John himself had indeed administered. According to Luke in the book of Acts, "then Paul said: 'John truly baptized with the baptism of repentance, while saying to the people that they should believe in Him Who would come after him' : i.e., in Christ Jesus" the Spirit-anointed One.
"When they heard [and heeded] this, they were baptized into the Name of the Lord Jesus." This could mean Paul was here simply saying that John himself274 had indeed baptized people into the Name of the then-coming Messiah — that is, the One Whom Paul here identifies as the Lord Jesus. Or it could mean Paul himself now baptized the unitarians at Ephesus — into the Name of the Lord Jesus. Either way, there is no re-baptism at Acts 19:5. For that verse says "they were baptized" — not: 'they were re-baptized.'
The fact is, 'Johannine baptism' is Christian baptism! Thus, specifically on this passage, nearly all Calvinist scholars. Thus Calvin, Beza, the 1637 Dordt Dutch Bible, Wolleb(ius), Lightfoot, Cocceius, Marckius, De Moor, J.H. Heidegger, J.H. van der Palm, H. Heppe, Gravemeijer, and A. Kuyper Sr.274 etc.
The text could mean that Paul was here informing the unitarians at Ephesus about what John himself275 had really taught. This would then show that those who had heeded John's preaching — John's preaching that they should believe in Jesus — were there and then baptized by John into the Name of the Lord Jesus. In that case, after explaining this to the confused men in Ephesus, all that Paul then further did — after they heeded him — was 'waterlessly' to lay his hands upon those ex-heretics.
Alternatively, the above words — "when they heeded this, they were baptized into the Name of the Lord Jesus" — may instead be referring to what those ex-unitarians in Ephesus next did in relation to Paul. This would mean that Paul himself then proceeded to give those ex-unitarians inter alia their first-ever triune water-baptism.
Perhaps Paul did not then give water-baptism to those men; on the other hand, it seems perhaps more likely that he did.275 Either way, however, the entire passage Acts 19:1-5 cannot properly be taken to mean that the trinitarian Paul re-baptized those ex-unitarians after they had heeded his teaching. If Paul then indeed baptized them with water, that would have been the first and the only Christian baptism those previously ignorant unitarians ever received!
For Paul explained to the unitarians at Ephesus that "John truly baptized with the baptism of repentance/” John did this, said Paul, “while saying to the people that they should believe in Him Who would come after him — that is, in Christ Jesus."
Whichever way the passage is taken, it cannot righty be taken to imply baptismal regeneration. Indeed, at whatever point in time that never-repeated water-baptism took place or was to take place in respect of the approximately twelve men mentioned in Acts 19:1-7 — it was not the same time at which their Christian faith commenced!
For any Christian water-baptism ever received by those ex-unitarians, would only have occurred after they had been regenerated by grace and through faith — and apparently as a result of Paul's preaching the Gospel to them. Previously, it seems they had never truly been baptized by anyone. If they then ever received Christian baptism at all — which indeed seems very likely — that could have occurred only after they heeded and obeyed the Gospel then preached to them by Paul.
That would then have been the first and the only Christian baptism those previously ignorant ex-unitarians had ever received. More importantly, they had already been regenerated — before they would then have received that baptism. Indeed, they were regenerated apparently while hearing the Gospel preached to them by Paul. Acts 19:4. They were therefore not regenerated by the Christian baptism itself, which would only be administrable to them at a somewhat different moment. Acts 19:5.