John Chrysostom

"And it came to pass, that, while Apollos was at Corinth, Paul, having passed through the upper coasts" - meaning what we have read as to Caesarea and the other places - "came to Ephesus, and having found certain disciples." But whence had those, being in Ephesus, the baptism of John? Probably they had been on a visit at Jerusalem at the time of John's preaching, and did not even know Jesus.

Homily on Acts 40 ↗

Theophylact of Ohrid

These, from among "certain disciples," who did not even know that there is a Holy Spirit, differed greatly from the other disciples, because from the words "that they should believe in Him who was coming after him" it is evident that they did not believe in Christ either. He did not say that the baptism of John was nothing, but that it was incomplete. How then, being in Ephesus, did they receive the baptism of John? …

Commentary on Acts ↗

Oecumenius

Knowing only John's baptism. It is worth asking how, if he knew only John's baptism, he nevertheless possessed the Spirit; for the Spirit was not given in that way. And if those who came after him needed the baptism of Christ, much more would he have needed it. What then is one to say? For he did not simply set both side by side. It seems, therefore, that he was one of the one hundred and twenty who were baptized after the apostles. [CHRYSOSTOM]

Commentary on Acts ↗

Cassiodorus

"And it came to pass, while Apollo was at Corinth, that Paul having passed through the upper coasts, came," etc. While Apollo was at Corinth, it came to pass that Paul, having passed through the upper coasts, came to Ephesus, where, having found certain disciples, he asked if they had received the grace of the Holy Ghost when they were baptized. They declared that they were entirely ignorant of that name, but had been consecrated in John's baptism. …

Complexiones on the Acts of the Apostles ↗