24 Now a Jew named Apollos, a native of Alexandria, came to Ephesus. He was an eloquent man, competent in the Scriptures. 25 He had been instructed in the way of the Lord. And being fervent in spirit,  he spoke and taught accurately the things concerning Jesus, though he knew only the baptism of John. 26 He began to speak boldly in the synagogue, but when Priscilla and Aquila heard him, they took him and explained to him the way of God more accurately. 27 And when he wished to cross to Achaia, the brothers encouraged him and wrote to the disciples to welcome him. When he arrived, he greatly helped those who through grace had believed, 28 for he powerfully refuted the Jews in public, showing by the Scriptures that the Christ was Jesus. (Acts 18:24-28 (ESV)
“Though he knew only the baptism of John.” It’s a curious thing, in some way Apollos had come to believe in Jesus Christ, and even spoke, that is preached, and by all accounts he taught accurately, that which he taught. More than that, he was a highly educated man and very eloquent. It seems he taught the truth, but not the whole truth. Evidently, not knowing the baptism of Christ he did not know the whole truth.
This serves also to show that John’s baptism was not Christian baptism, but something very different from what Christ would baptize with, as John himself proclaimed. Therefore, Christ’s baptism by John doesn’t serve as a model for Christian baptism.
Now Priscilla and Aquila hear Apollos in the synagogue of Ephesus. They had to have been shocked, amazed and comforted to hear Apollos speak, but they are quick to note that his teaching is deficient. The text doesn’t come out and say what it was that was lacking from Apollos’s teaching, yet one can imagine that the most obvious is that Apollos didn’t teach concerning baptism, which is pretty central to Christian teaching and proclamation as can be seen by Peter’s first sermon at Pentecost. It is in baptism that the gospel is applied to the subject and they become Christians.
Priscilla and Aquila, Paul mentions Priscilla first because she seems to be the most influential where this is concerned, they take Apollos to the side and instruct him more accurately concerning the Christian faith. Apollos himself seems to have received this instruction, even from a woman, humbly. This made him all the more powerful a preacher and an encouragement to the saints both in Ephesus and in Achaia. Both the fact that Paul praises Aquila so often, and that Apollos so willingly accepted instruction from her, show the Christian openness to women that was not so well known among Jewish circles. A willingness to accept truth no matter what its source.This should also be brought into account when in other letters Paul prohibits women from teaching men or speaking in the churches. It is notable that Priscilla and Aquilla took Apollos aside, outside the context of worship, which is the context in which Paul prohibits women from speaking or teaching. Sometimes these verses are divorced from their context and become nothing more than a cover for misogyny. But this was not only on account of Christian teaching concerning the role of women in worship, but also in consideration that reproof is often more ably accepted in private then in front of a crowd where one may lose more face than he is comfortable losing, by accepting the reproof.