What does Acts 20:21 mean?

Paul and the elders of the church in Ephesus are in nearby Miletus. Paul is giving them one last admonition before his final farewell. He reminds them of his service to them for the three years he lived in Ephesus, planting and building up the church. He boldly taught them "anything that was profitable" (Acts 20:20). Here, he gives three significant points.

First, he taught Jews and Gentiles (Acts 19:10); in fact, when Paul writes to the church in Ephesus, he seems to indicate the church is primarily Gentile (Ephesians 2:11; 3:1). We tend to miss the significance of this. In the early days of the church, Jews had a difficult time accepting that Jesus' salvation is available to everyone. The Holy Spirit led the apostles—particularly Peter—through a series of encounters that set the stage. Jews and very devout Gentiles came to faith in Jesus the day the Holy Spirit started the church (Acts 2:5, 11). Samaritans and an Ethiopian eunuch accepted Jesus later (Acts 8:14, 27). Then a Jewish enemy and persecutor of the church (Acts 9:1–19). Next, a houseful of Gentiles who worshiped God but had not converted to Judaism (Acts 10:1–2, 44–45). Finally, a city of Gentiles who had no prior devotion to the Jewish God (Acts 19:1–20). Jews and Gentiles worshiping together was a significant change in worldview.

Second, Paul taught repentance to God. This is the baptism of John the Baptist (Mark 1:4). At the end of Paul's second missionary journey, he spent a very short time in Ephesus (Acts 18:19–21) before leaving Aquila and Priscilla there while he returned to Syrian Antioch. The couple met a powerful Jewish teacher, Apollos, who was a disciple of John's and taught people to repent of their sins. Priscilla and Aquila explained that the Messiah John heralded was Jesus of Nazareth. Apollos accepted their words and started preaching about Jesus in Ephesus and Corinth (Acts 18:24–28). When Paul returned to Ephesus, his first encounter was with twelve men who were also disciples of John's. They were his first converts in Ephesus (Acts 19:1–7).

Finally, Paul taught faith in Jesus. This was the piece Apollos and the twelve men were missing. Because they had the background in John's message about repentance and the coming Messiah, they were primed to accept Jesus as the Savior they were waiting for. Others either took more time or rejected their own Messiah (Acts 19:8–10).