What does Acts 20:29 mean?

When Paul entered the city of Ephesus, it was filled with demons, witchcraft, and idols. The city was home to the temple of Artemis; however, this was not the Greek goddess of hunting and the moon. Rather, it was a local fertility goddess given the name so the people could worship their native goddess while maintaining nominal loyalty to the Empire's gods. The Holy Spirit empowered Paul to perform mighty acts of healing and exorcisms to validate his message of repentance and acceptance of Jesus. He made such an impression, and so many people chose to follow Jesus, that the local idol-makers started a riot attempting to keep their businesses going (Acts 19:11–34).

A few months later, Paul meets with the elders of the church in Ephesus and warns them that their people will be vulnerable to false teachers—including some from their own congregation (Acts 20:30). There's no indication that Paul had specific knowledge of such a thing happening in Ephesus, but he did elsewhere. In Corinth, the church members broke into sects, some following Apollos, Peter, Jesus, or himself (1 Corinthians 1:10–17). In Galatia, legalistic Jewish Christians tried to convince the Gentile Jesus-followers they had to be circumcised and follow the Jewish calendar (Galatians 4:9–11; 5:2–12).

The Ephesian elders do well in this. Decades later, Jesus will praise the church for testing those who claim to be apostles but are false teachers. Sadly, Jesus also notes that they have lost their first love: of Him (Revelation 2:2–4). The church that was founded on repentance needs to return to that humility and remember what Jesus has saved them from (Acts 19:1–4, 17–20; Revelation 2:5). Ignatius will later write the church a letter indicating they did just that.