What does Ephesians 4:20 mean?

Paul quickly and strongly contrasts the sinful practices of unbelievers with what the Ephesians have been taught. Paul's instructions to the Ephesians make it clear that Christians are not meant for a life of sin and shame. The Greek words of this verse imply a strong, clear, and complete difference from one thing to another. Paul has just discussed the spiritual failures of non-believers—and all Christians are non-believers before they come to know Christ. The separation between what we were and what we are is profound, and it should be reflected in our lives.

Paul's lengthy stay in Ephesus included twelve men he led to Christ (Acts 19:1–7), three months of speaking at the synagogue (Acts 19:8), and speaking for two years in the hall of Tyrannus (Acts 19:9–10). Everyone in the area, Jews and Greeks alike, "heard the word of the Lord" (Acts 19:10). After many people in the city burned their books of magic arts, Luke notes, "So the word of the Lord continued to increase and prevail mightily" (Acts 19:20).

Paul eventually left due to persecution, leaving a sizable congregation who had been well trained in the gospel. These believers knew right from wrong, and did not learn Christ in a way which promoted sin. Paul's words here reminded them to live distinctly different lives from their surrounding society.