Acts 18 ends Paul’s second missionary journey. It also provides us with some points with which to date the journey.
Luke mentions that the emperor Claudius expelled all the Jews from Rome. The writer Suetonius mentions this edict and it likely dates from 49-50 A.D. The date then for Paul’s arrival in Corinth would be after the edict.
Luke also mentions Gallio. Gallio was the brother of Seneca the Younger who was the tutor of the boy Nero who would become emperor in 54 A.D. From an inscription at Delphi, we know that Gallio was the proconsul of Achaia in 53 A.D. and he only served one year. And so, the events of chapter 18 occur about that time.
Chapter 18 also introduces us to an idea that will be dealt with in more detail in chapter 19. Evidently, there were preachers in the ancient world who knew the teaching and baptism of John the Baptist. Apollos evidently knew that teaching, and, as far as it went, he knew the correct teaching about Jesus. But there was something he didn’t know. Under John’s baptism, people were made ready for Jesus when he came. Baptised for the forgiveness of sins, they would await the Messiah and when he came, would follow him and eventually, become the first citizens of the Kingdom of God. It’s why you don’t read of the Apostles being baptized by Jesus again.
But after Jesus came, this baptism would not do. It was performed in expectation of something that had already happened.
Seems like a small thing, right? And yet, it was important enough for Priscilla and her husband Aquila to “correct” his teaching to know the way of God more adequately. If God has spoken specifically about a matter, it’s important for us to know and practice it as He has willed.